A flooded cave in Madagascar’s Tsimanampetsotsa Nature Reserve is also the home of a recently discovered underwater graveyard filled with fossils of long-extinct species, some several thousand years old.

A team of scientists led by Brooklyn College professor Alfred Rosenberger has discovered remarkably well-preserved fossils of turtles, rodents, crocodiles, and the elephant bird, an extinct flightless bird similar to an ostrich.

“We have a real cross-section of tiny things and big things,” Rosenberger told National Geographic. The big things in this case are the cave’s largest supplier of bones, extinct giant lemurs.

These predecessors to today’s lemurs lived up to their name, with some as large as gorillas. By comparison, the lemurs we know and love now range in weight from about an ounce up to 20 pounds. These giant primates went extinct between 2,000 and 500 years ago for reasons researchers are still trying to figure out.

How these bones ended up in the cave is another mystery. Rosenberger thinks that many washed up in the cave over time, before and after humans found their way to Madagascar.

Aven is one of three flooded caves in the park that researchers are exploring, and it may be Madagascar’s largest fossil graveyard. When the team first began exploring, the cave was so dense with bones that “divers felt them every time they put their hands down.” The Madagascar Cave Diving Association has recently set up an underwater museum to display some of the finds, and if you are an experienced diver, you can explore this submerged wonder.