Volcanoes, whether active, dormant, or extinct, are among the most awe-inspiring and volatile features of nature. They shape the earth's landscape, create new islands, and provide rare diving opportunities for those adventurous enough to explore their depths. One of the most volcanically active countries in the world is Indonesia. What makes Indonesia unique is not just the frequency of eruptions, but the fact that its volcanic regions are surrounded by some of the richest marine ecosystems on Earth.
North Sulawesi, located in Indonesia’s far northern archipelago, offers divers a rare and thrilling chance to dive near and around volcanoes. This region is home to a trifecta of volcanoes, each offering a different and unique underwater experience. These include Mahengetang, an active underwater volcano; Ruang, an active land volcano with extensive lava flows beneath the surface; and an extinct volcanic site that harbors a completely different geological and marine profile.
This part focuses on the thrilling experiences and challenges of diving Mahengetang—the active underwater volcano—and the striking beauty of the Ruang Volcano and its underwater lava gardens.
Diving Mahengetang: The Living Underwater Volcano
Mahengetang is the most difficult of the three volcanoes to dive. It is not just its remote location that presents a challenge, but also the extreme underwater conditions that surround it. Strong currents, high temperatures, thermoclines, and low visibility make this a site for experienced divers only. However, for those willing to take on the challenge, Mahengetang rewards with a unique and surreal environment found nowhere else in the world.
Situated near a small island, the Mahengetang underwater volcano sits beneath shallow waters and must be dived during slack tide. The currents around the pinnacle are strong, changing rapidly with the tides, and often create whirling eddies and shifting flow patterns that test even the most seasoned divers.
Interestingly, a scientific expedition a few years ago reported that the volcanic peak was five meters below the surface. On our visit just four years later, we discovered the peak had risen significantly and was now protruding above the surface, a clear sign of the volcano’s ongoing geological activity. This transformation is a reminder of the living, breathing nature of the site.
Sulphur Bubbles and Scalding Heat
Descending into the waters around Mahengetang feels like entering another world. Surrounding the rocky peak are countless cracks and fissures in the sea floor that constantly release sulphur bubbles. These bubbles rise steadily, creating shimmering curtains in the water that distort vision and lend the dive an otherworldly quality.
The smell of sulphur is pungent even above the water. Once submerged, the heat becomes instantly noticeable. Water temperatures around the volcano average 35°C (95°F), which is far warmer than typical tropical diving locations. The heat makes long exposure uncomfortable, yet a lightweight wetsuit or exposure suit is still recommended to protect against cuts from sharp rocks and stinging marine organisms.
The high temperature affects not only the divers but also the marine life. The heat encourages the growth of unusually shaped soft corals and vibrant sponges that seem almost alien in form and color. These heat-tolerant organisms thrive here, clinging to the blackened volcanic rocks in twisting, unnatural patterns that make the reef look like something from another planet.
The Challenge of Underwater Photography at Mahengetang
Photographing Mahengetang is one of the most difficult tasks an underwater photographer can face. Thermoclines—steep temperature gradients that create visual distortion—begin at the surface and extend down to at least 35 meters. These layers bend and scatter light, creating swirling hazes that make it nearly impossible to achieve a sharp focus or clear exposure.
To add to the difficulty, available light is severely reduced. The bubbling sulphur, combined with heat distortions and particulate matter, cuts down sunlight significantly. The result is an underwater environment that is both dark and murky despite being relatively shallow.
Photographers must constantly adjust their settings—shifting ISO, aperture, and shutter speed on the fly to deal with the rapidly changing conditions. Shooting wide angle is especially difficult due to the inconsistent lighting and haze. On our initial dives, we used models wearing black exposure suits, which proved nearly impossible to photograph properly. The dark suits absorbed the limited light and became indistinguishable from the volcanic background. For later dives, we switched to brightly colored suits, which not only improved visibility in photos but also added an extra layer of safety in the low-visibility conditions.
Despite the many obstacles, those rare moments when the haze clears, or a sudden shaft of sunlight breaks through, offer stunning opportunities to capture the volcanic terrain in all its harsh beauty.
Surface Intervals with Island Life
After each dive at Mahengetang, surface intervals provide a unique cultural experience. The small nearby island offers a chance to stretch your legs and glimpse local life. Though the islanders do not speak English, they are incredibly welcoming.
Walking along the shore, we saw locals mending fishing nets and crafting large timber boats using little more than chainsaws and hand tools. Children laughed and followed us eagerly, curious about our gear and cameras. Their smiles and open hospitality provided a striking contrast to the intense and dramatic underwater environment we had just emerged from.
This slice of island life is a reminder of the human connection to these volcanic waters. The people who live here have adapted to the rhythms of the volcano, integrating its presence into their everyday lives.
Ruang: A Volcano Above and Below
If Mahengetang is a volcano that emerges from beneath the sea, Ruang is the opposite—a massive land volcano whose impact extends deep below the water’s surface. Traveling to Ruang by boat is breathtaking. The volcano forms a nearly perfect cone, its flanks dropping steeply into the sea, creating a circular island surrounded by deep, clear waters.
The most captivating aspect of Ruang is the two vast lava flows that descend from the peak and continue into the ocean. These flows are visible above water as jagged, black rivers of rock that cascade down the hillside. Underwater, they transform into dynamic reefs filled with life.
The rock is porous and rough, making it an excellent substrate for coral growth. Over the years, these lava flows have evolved into expansive coral gardens, home to both soft and hard corals, sea whips, fans, and an abundance of sponges. The vibrant colors and diversity of species make Ruang one of the most visually stunning dive sites in North Sulawesi.
Drift Diving Through Lava Reefs
Ruang’s underwater lava reefs are vast and sweeping, and the strong currents that flow over them make this an ideal site for drift diving. Once you enter the water and descend, the current carries you effortlessly across a panoramic landscape of coral and volcanic stone.
Unlike the challenging visibility of Mahengetang, Ruang’s lava gardens are relatively clear and well-lit. The black volcanic rock forms a dramatic contrast to the explosion of colors from the marine life. Schools of anthias, parrotfish, and wrasses dart in and out of coral formations, while larger species like groupers and bumphead parrotfish cruise the edges of the reef.
The dive site is not only beautiful but also dynamic. Small channels and valleys in the lava create unique pathways for the currents, which in turn sculpt the coral formations and influence where different species choose to live. It is an underwater world shaped not only by biology but also by geology and time.
A Macro and Wide Angle Paradise
The interplay of volcanic rock and marine life at Ruang creates a perfect environment for both macro and wide-angle photography. Between the large coral heads and rocky crevices are pockets of black sand, which provide an excellent hunting ground for macro subjects.
Tiny nudibranchs, shrimp, crabs, and gobies can be found hiding in these areas. The black sand makes for a perfect background that helps macro subjects stand out sharply in photographs. Meanwhile, wide-angle photographers will delight in the grand sweep of coral fields and the sheer cliff-like edges of the lava flows.
Every corner of this dive site holds something special, from the smallest creature crawling along the sand to the immense formations of coral encrusting the submerged lava.
Diving the Extinct Volcanoes of North Sulawesi
While diving active and recently active volcanoes offers the thrill of geological transformation in real-time, there’s something deeply humbling and mesmerizing about diving an extinct volcano. These submerged giants have long since quieted, but their legacy lives on beneath the waves. In North Sulawesi, extinct volcanic formations create habitats that have evolved into vibrant underwater ecosystems.
Diving these extinct sites is a journey into deep time, where layers of rock, coral, and sediment silently tell the story of millennia. Unlike the volatile environments of Mahengetang or Ruang, extinct volcanic sites offer calm waters, stable structures, and some of the richest marine biodiversity found anywhere in the Coral Triangle.
Geological Time Beneath the Surface
An extinct volcano has not erupted for tens of thousands, or even millions, of years and shows no signs of future activity. Over time, wind, rain, tides, and tectonic forces erode the original structure, softening jagged peaks and reshaping calderas. When these formations occur underwater or are eventually submerged by rising seas, they become substrates for marine colonization.
The lava rock cools and breaks down, creating surfaces that are ideal for coral polyps to attach and grow. What was once a bare volcanic outcrop gradually transforms into a reef, as sponges, corals, anemones, and other sessile organisms cover its surface. Over decades and centuries, fish and invertebrates take residence, forming a complete marine ecosystem.
In North Sulawesi, extinct volcanic structures are often located in deeper waters with gently sloping sides and valleys formed by ancient lava channels. These topographies provide shelter, feeding grounds, and nurseries for countless species, making them biological hot spots and fantastic dive sites for exploration and photography.
Submerged Calderas and Crater Lakes
One of the most intriguing aspects of extinct volcanic sites is the presence of submerged calderas and crater lakes. A caldera is formed when a volcano erupts so violently that its magma chamber collapses, leaving behind a depression. When located offshore, these calderas often fill with water and become diveable basins surrounded by steep walls.
Crater diving provides a different visual experience than reef or wall diving. The terrain is more rugged, and the visibility tends to be excellent due to the depth and clarity of the water. Sunlight filters down in shafts through the water column, illuminating ridges and canyons formed by ancient eruptions.
Some crater dives feature hydrothermal vents or fumaroles—residual signs of once active geological forces. Although these are no longer dangerous, they add an element of mystery and beauty to the dive. Fine mineral deposits around the vents form strange textures and colors that contrast with the surrounding coral.
Crater walls are often lined with plate corals, sea fans, and barrel sponges. As you descend deeper into the crater, the pressure increases, light decreases, and the marine life changes accordingly. At deeper depths, you may find deepwater species such as flashlight fish, soldierfish, and larger pelagics cruising the outer rim.
Marine Biodiversity on Ancient Lava Slopes
Lava slopes formed by extinct volcanoes are ideal locations for marine biodiversity to thrive. The porous and irregular surfaces trap nutrients and provide hiding places for small organisms. Over time, the slope transforms into a living reef composed of corals, sponges, tunicates, and a wide variety of algae.
Soft corals in pink, purple, and gold sway gently in the current, while hard corals form dense thickets that offer protection for juveniles and small reef dwellers. Crinoids extend their feathery arms to feed, and lionfish hover like sentries over their rocky domains.
Macro life abounds on these slopes. Frogfish, leaf scorpionfish, ghost pipefish, and ribbon eels are just some of the exotic finds that make macro photography here so rewarding. Brightly colored nudibranchs glide across the rock surfaces, and mimic octopuses occasionally make appearances in the sand and rubble at the base of the slope.
Divers who enjoy night dives will find the extinct volcanic sites especially rewarding after sunset. Crabs, lobsters, cuttlefish, and squid emerge from their crevices. Bioluminescent plankton flash briefly when disturbed, turning the water column into a galaxy of glowing lights.
Coral Colonies and Ecological Succession
One of the most fascinating processes visible at extinct volcanic sites is ecological succession—the gradual replacement of bare rock by increasingly complex forms of marine life. It begins with colonizing organisms such as bacteria and algae, which form thin biofilms. These are followed by barnacles, tube worms, and other early settlers.
Next come the coral polyps, which build calcium carbonate structures that grow and branch over time. Sponges fill in the gaps and filter the water. Eventually, a complete food web forms: herbivores graze on algae, predators hunt among the coral heads, and detritivores clean up the debris.
At mature sites, corals often compete for space. You’ll see evidence of coral warfare—branching corals encroaching on massive corals, and encrusting corals overtaking their slower-growing neighbors. The result is a reefscape that is not static but constantly evolving.
These extinct volcanoes, once centers of destruction, now play a critical role in marine conservation. They act as biodiversity refuges, supporting both endemic and migratory species. Their isolated nature protects them from some human impacts, allowing ecosystems to flourish with minimal interference.
The Role of Volcanic Sediments in Nutrient Cycling
Volcanic sediments play an essential role in enriching the marine environment. Unlike regular sand, which is mostly composed of silicates, volcanic sand and silt contain high levels of minerals such as iron, magnesium, and phosphorus. These nutrients are slowly released into the water column, supporting phytoplankton blooms and driving the food chain from the bottom up.
In regions like North Sulawesi, upwelling currents carry these nutrients into shallow waters where sunlight can trigger photosynthesis. This leads to greater productivity, more plankton, and ultimately more fish. Large schools of jacks, fusiliers, and trevallies can often be found near extinct volcano slopes, feeding on the abundance of life.
Even filter-feeding species such as manta rays and whale sharks are known to visit nutrient-rich volcanic sites. While sightings are never guaranteed, these gentle giants occasionally glide past divers at deeper reefs or during seasonal plankton blooms.
Access and Diving Conditions
Extinct volcano dive sites in North Sulawesi tend to be more accessible and forgiving than their active counterparts. Most can be reached within an hour or two by boat from regional dive hubs, and diving conditions are usually calm with good visibility.
Depths vary, but many of the best features begin at around 10 meters and descend to recreational diving limits of 30 meters. Further exploration beyond this requires technical diving certification due to the depth and potential decompression risks.
Water temperatures range between 27°C to 30°C, making standard wetsuits ideal for long dives. Currents are generally mild, though some areas can experience stronger drift depending on the tides. Dive planning is still essential, especially when diving calderas or steep drop-offs.
Safety protocols should always be followed. Dive briefings often include topographic maps, current predictions, and possible entry and exit points. Because these sites are sometimes isolated, having an experienced dive guide and proper communication equipment is recommended.
Photography Opportunities on Ancient Reefs
From a photography standpoint, extinct volcanic sites offer some of the most stable and consistent lighting conditions compared to their active counterparts. Without thermoclines, haze, or bubbling gas, the water is clear and calm, ideal for both macro and wide-angle work.
Underwater photographers will appreciate the contrast between the dark volcanic rock and the bright marine life. Lava tubes, arches, and collapsed crater walls offer dramatic framing opportunities for wide-angle shots, especially when sunlight filters in from above.
Macro photographers will find no shortage of subjects. The rich sediment and coral mix is a haven for unusual invertebrates. Focus peaking, manual strobes, and higher aperture settings help capture the intricate textures of tiny creatures in crisp detail.
For those who enjoy blackwater or night photography, extinct sites provide a relatively safe and rewarding setting. Pelagic larvae, jellyfish, and ctenophores are often found in the water column after dark, revealing a whole new world of photographic possibilities.
Marine Conservation and Volcanic Reefs
The biological richness of extinct volcanic reefs makes them important zones for conservation. Because these reefs often exist far from population centers, they may be spared from some of the worst effects of coastal development, overfishing, and pollution. However, they are not immune to threats.
Climate change and ocean acidification still pose significant risks. Coral bleaching, shifts in species distribution, and storm damage are all challenges that even these remote reefs face. Conservation efforts in North Sulawesi include marine protected areas, community-led reef monitoring, and education programs for sustainable tourism.
Divers can play a role in preserving these ecosystems by following best practices: avoiding contact with corals, using reef-safe sunscreen, minimizing plastic use, and supporting eco-conscious dive operators. Responsible tourism helps ensure these beautiful sites remain intact for future generations.
Human Connection to Volcanic Reefs
Communities living near extinct volcanic sites have deep ties to these underwater landscapes. For many, these reefs are not just dive destinations—they are sources of food, cultural heritage, and local identity. Fishermen use traditional methods to harvest sustainably, and elders pass down knowledge of tides, fish behavior, and reef geography.
In recent years, some villages have embraced ecotourism as a way to protect their resources while providing economic opportunities. Local guides, boat operators, and dive assistants take pride in showcasing the underwater beauty of their homeland, creating a meaningful exchange between visitors and hosts.
By diving extinct volcanoes, you not only explore a geological relic—you also engage with a living, evolving ecosystem shaped by nature and nurtured by people.
The Science Behind Volcanic Dive Sites
Diving around volcanoes isn't just about adventure; it's an invitation to witness Earth's most powerful natural processes in action. The science behind these environments is both fascinating and vital to our understanding of the planet. When tectonic plates collide or drift apart, magma rises to the surface, creating underwater and terrestrial volcanoes. Over time, these structures form islands, seamounts, and geothermal vents that support diverse ecosystems.
Hydrothermal Vents and Unique Ecosystems
One of the most intriguing features in volcanic dive sites is the presence of hydrothermal vents. These are cracks in the ocean floor that emit superheated, mineral-rich water. The temperatures can exceed 400°C, but due to the high pressure at depth, the water remains in a liquid state. Around these vents, life thrives in forms not seen elsewhere on Earth. Instead of relying on sunlight, organisms like tube worms and thermophilic bacteria depend on chemosynthesis—a process where bacteria convert chemical energy from the vent fluids into food.
This alien-like biodiversity underscores the adaptability of life and has implications for astrobiology, as it raises the possibility of similar ecosystems existing on icy moons like Europa or Enceladus, where subsurface oceans may harbor hydrothermal activity.
Lava Formations and Geological Marvels
Divers often witness the dramatic aftermath of underwater eruptions. Lava tubes, pillow lavas, and basaltic formations are common in volcanic regions. These structures offer shelter for marine life, creating a dynamic landscape where geology and biology converge.
Lava tubes—formed when molten lava flows beneath the surface and then drains away—create long, hollow tunnels that divers can explore. Pillow lava, characterized by bulbous, rounded shapes, forms as lava cools rapidly upon contact with seawater. These formations not only tell a geological story but also serve as nurseries for juvenile fish and invertebrates.
Seismic Monitoring and Volcanology Underwater
Diving near volcanoes also allows scientists and trained technical divers to place monitoring equipment in strategic locations. Seismometers, gas sensors, and cameras help researchers track activity, predict eruptions, and better understand underwater volcanic behavior.
In places like the Santorini caldera or the submerged summit of Kick ’em Jenny near Grenada, research dives provide critical data for early warning systems and contribute to hazard mitigation for nearby coastal communities.
Preservation of Volcanic Marine Environments
As volcanic dive tourism grows, so does the responsibility to protect these fragile ecosystems. The same geothermal heat and mineral richness that attract life can also be easily disrupted by human interference.
Responsible Tourism Practices
To ensure sustainable diving around volcanoes, dive operators and tourists must follow strict guidelines. Anchoring should be avoided to prevent damage to coral and rock structures. Divers are encouraged to maintain buoyancy control to avoid touching or stirring sediment on sensitive formations.
Limiting the number of daily divers, setting up marine protected areas (MPAs), and creating no-dive zones around especially fragile hydrothermal vent systems are all part of preserving these habitats for future generations.
Education also plays a major role. By briefing divers about the unique geological and biological features of volcanic environments, dive guides can instill a sense of respect and stewardship for these rare ecosystems.
Climate Change and Environmental Impact
Volcanic environments, like all marine habitats, are vulnerable to the effects of climate change. Ocean acidification, rising sea temperatures, and pollution can all compromise the delicate balance of life near underwater volcanoes.
Coral reefs that thrive around volcanic islands, such as those in Indonesia, Hawaii, or the Galápagos, are at particular risk. Warmer waters can lead to coral bleaching, while stronger storms and shifting currents can alter the distribution of nutrients and marine populations.
Volcanic environments that were once pristine are increasingly exposed to plastic pollution and overfishing. Addressing these challenges requires global cooperation, effective marine policies, and local conservation efforts.
The Future of Volcanic Diving
The future of diving in volcanic regions holds immense promise. As interest grows and technology advances, new sites are being discovered, and old ones are being explored with greater depth and understanding.
Technological Innovations in Dive Exploration
Remotely operated vehicles (ROVs), autonomous underwater drones, and mixed-gas rebreathers now allow divers and researchers to reach depths previously inaccessible. These technologies enable longer dive times, reduced nitrogen absorption, and real-time video transmission for both scientific and recreational purposes.
Photogrammetry, sonar mapping, and 3D modeling are being used to digitally recreate lava tubes, hydrothermal fields, and ancient submerged calderas. These tools allow for non-invasive study and can also enhance educational and virtual tourism experiences.
Citizen Science and Community Involvement
The diving community has increasingly become involved in citizen science. Divers visiting volcanic sites can contribute to research by photographing marine life, recording observations of unusual activity, and collecting data through dive logs. Apps and online platforms are now available for divers to upload their findings, which are then analyzed by marine scientists.
Local communities that live around volcanoes—such as those in Vanuatu, the Philippines, or Iceland—also play an essential role in marine preservation. Sustainable tourism, community-led conservation programs, and dive training for local youth create economic opportunities while fostering environmental awareness.
Volcanic Dive Destinations: The Next Frontier
Several volcanic dive destinations are emerging as world-renowned hotspots. Here's a brief glimpse into what makes some of them stand out:
Milne Bay, Papua New Guinea
An area of incredible biodiversity and volcanic terrain, Milne Bay features black sand dive sites, lava tubes, and hidden caves. The convergence of Pacific and Australian tectonic plates creates both stunning topography and fertile grounds for coral and macro life.
Banda Sea, Indonesia
The Banda Sea is home to both active and extinct volcanic islands, offering dramatic walls, crystal-clear waters, and encounters with hammerhead sharks. The Ring of Fire's restless geology ensures there's always something new to discover.
Savo Island, Solomon Islands
This volcanic island features both hot springs and geothermal bubbles underwater. The diving is a mix of reef, muck, and volcanic exploration, offering unique underwater landscapes shaped by constant geothermal shifts.
La Palma, Canary Islands
Known for recent volcanic activity, La Palma combines lava-formed underwater arches and tunnels with Atlantic marine life. Recent eruptions have reshaped parts of the coast, offering a rare look at post-eruption marine colonization.
Kīlauea, Big Island, Hawaii
Although recent lava flows have made certain areas inaccessible, diving along the lava coast reveals arches, craters, and formations where fire met the sea. Marine life here is as resilient as the terrain.
Conclusion:
Diving with volcanoes is a transformative experience that brings together the thrill of exploration, the beauty of nature, and the awe-inspiring power of Earth's inner forces. From hydrothermal vents teeming with alien life to black sand slopes hiding rare species, these sites offer something that few other dive environments can—constant change and raw beauty born of fire.
But with this opportunity comes responsibility. As divers, scientists, and travelers, we are guests in one of nature’s most dynamic habitats. By diving responsibly, supporting sustainable tourism, and sharing our experiences, we help ensure that future generations can also explore these mystical worlds where molten rock meets ocean depths.