From the outside, Mare aux Songes looks completely harmless—a grassy swampland in the middle of Mauritius, a beautiful island oasis some 500 miles off the coast of Madagascar.

Scientists have deemed it a Lagerstätte, German for “storage space.” That’s because well-preserved fossils litter the vicinity of this one-time lake, which provided the only source of fresh water for native species during an extensive drought that lasted 50 years.

Erik de Boer, a paleoecologist at the University of Amsterdam, wanted to know why the ancient lake—which should have saved these animals—instead became the source of their demise. He and a team of experts reconstructed the history of the island’s ecosystem by analyzing sediments taken from the area today.

They concluded that decreased monsoon activity caused the drought, but at the same time, the lake became increasingly muddy, salty, and, well, poopy. In fact, during the island’s dry season, it was essentially undrinkable. Here’s David Schultz, writing for Science: