Between Africa and the island of Madagascar is the Comoros archipelago, and in May 2018, the easternmost island of Mayotte was struck with a bunch of earthquakes. This cluster came as quite a shock to the residents of the island, which only had one seismic station at the time. Investigations by French scientists from the Institut de Physique du Globe de Paris revealed a new submarine volcano had developed about 50 kilometers off the coast of the tiny island. The results were presented at AGU’s Fall Meeting.

Then those scientists determined that this particular submarine eruption was the largest in documented history. Keep in mind that we haven’t actually documented a lot of underwater eruptions in their entirety, but this eruption is still huge and is still going.

As the team has collected more and more data, and more seismic stations were installed, a clearer and more detailed picture of the submarine structures has been created. And this is where it gets a little weird. You see, there are two clusters of earthquakes — one along the ridge where the new volcano is and one in a donut shape that implies the existence of a caldera. They are not related in a “connected to each other” kind of way, and in fact, the caldera appears to be a much older structure.

Even stranger, the fracturing around the caldera is located at a depth where brittle rock generally doesn’t exist because it’s too warm and more plastic than at the level of the ocean floor. This concerns scientists because if they cannot understand it, they cannot make predictions about its behavior, and the caldera is closer to the island than the volcano. Scientists are now concerned that this caldera is an untapped magma source slowly moving upward through the lithosphere, and its existence could signal a significant threat for the nearby population.

Obviously, the team will continue to monitor the seismicity of the region, and more observations are needed to determine what exactly is happening.

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