The research team, led by Marta Ribó and colleagues, compared high-resolution bathymetric surveys of the Hunga caldera acquired before and after the eruption. This before-and-after approach allowed them to quantify the morphological change with unusual precision. The inner caldera floor, which had sat at a depth of roughly 150 to 250 metres below sea level prior to the eruption, was found to have deepened dramatically, with the central portion of the volcano dropping by over 1,000 metres. The volume of material displaced during this structural failure, calculated at 8.9 cubic kilometres with an uncertainty of only 0.1 cubic kilometres, ranks among the largest caldera collapses ever documented in the modern instrumental era.

Crucially, the geophysical surveys revealed that the collapse was not a slow, post-eruptive settling process but a rapid event that unfolded during the climactic eruption itself. Seismic records from the eruption show phases of intense ground shaking, and the new morphological evidence indicates that the roof of the magmatic system foundered as enormous quantities of magma were evacuated from the reservoir below. This synchronisation between magma discharge and structural collapse is the key to understanding why the 2022 Hunga eruption was so exceptionally hazardous. As the caldera floor dropped, seawater rushed into the newly opened volcanic conduit system, contacting magma at shallow depths and generating steam on a colossal scale.

That steam generation explains one of the eruption’s most puzzling features: the extraordinary height of its plume. Subsequent work by Mastin and colleagues, cited in the new study, demonstrated that intense seawater–magma interaction boosted the height and growth rate of the giant Hunga eruption plume well beyond what the magma’s thermal energy alone could achieve. The plume injected material into the stratosphere and even into the mesosphere, an altitude reached by no other volcanic eruption observed with modern satellite instrumentation. The new caldera-collapse reconstruction provides the physical context for this process, showing that the collapsing roof structure continuously opened pathways for seawater to reach the erupting magma.

The collapse also helps explain the peculiar chemistry of the eruption. Work by Wu and colleagues, published in Nature Geoscience in 2025, showed that sulfur emissions from the eruption were surprisingly low, and concluded that magma fragmentation occurred below sea level during the climactic phase. A submarine fragmentation regime, sustained by the rapid foundering of the caldera floor, would suppress the release of sulfur-rich gases into the atmosphere while simultaneously powering the steam-driven explosivity. In other words, the depth at which the magma disintegrated, controlled by the collapsing caldera structure, shaped both the atmospheric consequences and the climatic footprint of the eruption, which were far smaller in terms of sulfate aerosol loading than the eruption’s explosive violence would have suggested.

Perhaps the most destructive consequence of the collapse unfolded along the seafloor itself. As the caldera walls failed and vast volumes of volcanic debris were mobilised, fast-moving underwater density currents raced down the flanks of the volcano. Research by Clare and colleagues in Science documented how these volcaniclastic density currents, triggered by the ocean-entering eruption, travelled tens to hundreds of kilometres and inflicted extensive damage on submarine infrastructure. Several critical telecommunication cables connecting Tonga to the global internet were severed, isolating the kingdom for weeks. The new study ties these currents directly to the caldera collapse, showing that the sudden displacement of 8.9 cubic kilometres of material provided the sediment supply and gravitational energy needed to sustain such fast and far-travelling flows.

The tsunami hazard, which claimed lives as far away as Peru, also appears to have been driven in large part by the collapse dynamics. While the eruption generated atmospheric pressure waves that produced tsunami-like oscillations across the entire Pacific basin, the near-field waves that devastated coastlines in Tonga were far larger than atmospheric forcing alone could explain. A rapid caldera collapse displaces seawater in much the same way as a landslide, generating impulsive local waves. The timing established by the new surveys, with the collapse occurring during the climactic eruption, supports the interpretation that the foundering of the volcano’s interior was a primary source of the extreme local tsunami runup documented by Borrero and colleagues along the coasts of Tonga.

The study also contributes to understanding the magmatic plumbing system beneath Hunga. Complementary work by Le Mével and colleagues characterised the magmatic reservoir before and after the 15 January 2022 eruption, documenting the changes induced by the evacuation of magma. The new morphological data provide the surface expression of that subsurface drainage: the more than one kilometre of collapse directly reflects the volume of magma withdrawn from the reservoir and the mechanical failure of the rock above it. Together, these datasets allow scientists to close the loop between what happened at depth, what erupted at the seafloor, and what propagated through the ocean and atmosphere.

Beyond Hunga, the findings carry significant implications for hazard assessment at submarine volcanoes worldwide. Roughly three-quarters of Earth’s volcanic activity occurs underwater, yet submarine caldera collapses have rarely been captured with sufficient seafloor data to quantify their dynamics. The Hunga case demonstrates that a single collapse event can simultaneously generate multiple extreme hazards: a stratospheric eruption plume, devastating local tsunamis, ocean-basin pressure waves, and seafloor density currents capable of destroying critical infrastructure. The study underscores the value of repeat seafloor mapping, which made it possible to measure the collapse directly rather than inferring it from indirect evidence, and suggests that monitoring submarine caldera systems should be a priority for protecting coastal communities and the submarine cable networks on which global communications depend.

As researchers continue to analyse the wealth of data generated by the 2022 eruption, the picture that emerges is of a volcano whose most violent behaviour was governed by structural failure. The sudden collapse of Hunga’s interior was not merely a consequence of the eruption but an active driver of its hazards, amplifying the plume, the tsunami, and the underwater currents that made the event one of the most remarkable natural phenomena of the century. The Nature Geoscience study provides the quantitative foundation for that understanding, and a sobering reminder of how much destructive power can be unleashed when the seafloor itself gives way.

Subject of Research: Rapid submarine caldera collapse during the 2022 Hunga volcano eruption in Tonga and its associated extreme hazards

Article Title: Sudden collapse of submarine volcano drives extreme hazards

Article References: Sudden collapse of submarine volcano drives extreme hazards. (2026). Nature Geoscience. https://doi.org/10.1038/s41561-026-02098-8

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02098-8

Keywords: Hunga volcano, caldera collapse, submarine volcanism, tsunami, Tonga, eruption plume, density currents, seafloor mapping, natural hazards, magma, Nature Geoscience, underwater cables