Beneath the turbulent surface of the open sea, where sunlight fades and the pressure mounts, lie some of the most powerful and mysterious forces on Earth: submarine volcanoes. Invisible to most of us, they have long been the domain of geologists and deep-sea explorers. In recent years, however, they have stepped decisively into the spotlight of global maritime affairs.
From the cataclysmic eruption of Hunga Tonga-Hunga Ha’apai to the slow, unfolding crisis at Mayotte, the world has begun to recognise that the ocean floor is not just a passive backdrop for shipping lanes and fisheries. It is an active, dynamic landscape that can reshape economies, disrupt communications, and challenge the safety of those who live by the sea.
A world beneath the waves
A submarine volcano is, by definition, an active or ancient volcanic edifice located below sea level. These structures form along mid-ocean ridges, in subduction zones, above hotspots, or within island arcs. They may take the form of towering cones, broad shields, or elongated rift systems, often capped with craters and fed by networks of lava flows and hydrothermal vents.
What makes them so compelling is their paradoxical nature: they are both hidden and hugely influential. While most remain out of sight, the majority of Earth’s volcanic activity actually takes place beneath the ocean. Here, molten rock meets cold seawater, creating new crust, feeding strange ecosystems, and occasionally sending shockwaves that ripple across the planet.
Tonga: A wake-up call for island nations
The eruption of Hunga Tonga-Hunga Ha’apai on 15 January 2022 was a stark reminder of what lies beneath. The explosion sent a towering plume into the stratosphere, triggered a trans-Pacific tsunami, and generated atmospheric shock waves that circled the globe.
The impact on Tonga was devastating. According to a World Bank assessment, the direct damages were estimated at US$90.4 million roughly 18.5 percent of the country’s GDP. Ports were damaged, homes were buried in ash, and the country’s sole undersea cable was severed, isolating it from the rest of the digital world.
For maritime nations, the message was clear: geology is not confined to the land. An eruption deep beneath the waves can destabilise entire island economies overnight.
Mayotte: A slow-motion crisis
If Tonga was sudden and dramatic, the crisis at Mayotte has been measured, persistent, and deeply unsettling. Since May 2018, the French overseas territory in the Indian Ocean has been shaken by thousands of earthquakes. Residents reported feeling the ground tremble day and night, yet the source of the unrest remained invisible, hidden beneath the ocean.
In 2019, scientists identified a new submarine volcano approximately 50 kilometres east of the island. Named Fani Maoré, it sits at a depth of around 3,500 metres and has become the focus of intense study. The REVOSIMA network was established to monitor the seismic and volcanic activity, provide scientific data, and inform the public.
Mayotte’s experience highlights a different kind of risk: one that unfolds over months or years, testing the resilience of communities and the credibility of institutions. It is a reminder that not all volcanic crises are short and explosive. Some are slow, quiet, and deeply unpredictable.
The sea as a hazard zone
The dangers of submarine volcanoes extend far beyond tsunamis. An eruption can disrupt shipping, generate pumice rafts that clog harbours, alter local water chemistry, and even threaten the safety of vessels. In the Caribbean, Kick ’em Jenny a submarine volcano north of Grenada is closely monitored by the UWI Seismic Research Centre. When activity increases, maritime exclusion zones are put in place to protect ships and small craft.
Crucially, the greatest danger is often not to populations on land, but to those at sea: fishermen, cargo vessels, cruise ships, and scientific expeditions. A zone of caution around a submarine volcano may not signal an imminent threat to cities, but it can fundamentally alter maritime operations.
The fragility of digital lifelines
In the modern world, the ocean is also the backbone of global communications. Undersea cables carry the vast majority of international data traffic, linking continents, islands, and financial markets. These cables lie on the ocean floor, often in close proximity to volcanic zones.
The Tonga eruption demonstrated how fragile this infrastructure can be. When the cable was cut, the country lost most of its international connectivity. Recovery was slow, costly, and dependent on satellite alternatives with limited capacity.
Submarine volcanoes are no longer just a geological curiosity. They are now a strategic concern for governments, telecom operators, insurers, and anyone who relies on a connected world.
Laboratories of life
Beyond their hazards, submarine volcanoes are among the most extraordinary places on Earth. Around deep-sea hydrothermal vents, life thrives in conditions once thought impossible. Organisms survive without sunlight, drawing energy from chemical reactions in superheated fluids. These ecosystems have transformed our understanding of biology, chemistry, and the origins of life itself.
They also represent a new frontier for science and industry. Hydrothermal systems and seamounts are often rich in rare minerals, including polymetallic sulfides and cobalt-rich crusts. As land-based resources become scarcer, interest in deep-sea mining has grown.
Yet the debate is intense. Scientific and environmental communities warn that these ecosystems are fragile and poorly understood. Opening them to industrial extraction could have irreversible consequences – or it could unlock resources essential for the energy transition. The choice is not simple.
A new way of thinking about the Ocean
The rise of submarine volcanoes in public discourse signals a broader shift. The ocean is no longer seen only as a space of trade, transport, and leisure. It is also a space of risk, knowledge, and sovereignty.
Monitoring submarine volcanoes is no longer just a task for volcanologists. It involves maritime authorities, cable operators, port authorities, fisheries, insurers, coastal governments, and international organisations. It requires investment in technology, long-term observation, and a commitment to transparent communication.
The challenge is not to dramatise these phenomena, but to understand them. Submarine volcanoes can be dangerous, but they are also essential to the formation of the oceanic crust, the biodiversity of the deep, and the future of ocean science.
In the end, the story of submarine volcanoes is the story of how much of our maritime security is played out in invisible spaces far from the coast, where observation is costly, intermittent, and deeply strategic.
