Smithsonian/Corey Howell A 2004 photo of Kavachi erupting

Ocean explorers recently found sharks at the bottom of Kavachi, one of the most active submarine volcanoes in the Pacific. The waters in and around the volcano are too hot and acidic for human divers to swim—even when the crater is not spewing ash, lava, and steam up to the ocean’s surface—so oceanographer Brennan Phillips and his colleagues deployed disposable robots and cameras into the deep to explore the site, which is located near the Solomon Islands.

Phillips’s expedition was partly sponsored by the National Geographic Society and documented by a National Geographic video crew. In the footage below, the scientists cheer at their first glimpse of a stingray inside one of Kavachi’s ash plumes. They’re even more excited when a hammerhead shark comes into view.

They ended up seeing multiple hammerheads and silky sharks. “The idea of there being large animals like sharks hanging out and living inside… this volcano conflicts with what we know about Kavachi, which is that it erupts,” Phillips says in the video. “But when it’s erupting, there’s no way anything could live in there. So to see large animals like this that are living—and potentially, you know, they could die at any moment—it brings up lots of questions: Do they leave, do they have some sort of sign that it’s about to erupt? [Or] do they blow up sky high in little bits?”