Nine hundred kilometers off the east coast of Madagascar lies the tiny island paradise of Mauritius. The waters are pristine, the beaches bright white, and the average temperature hovers between 22°C and 28°C (72°F to 82°F) year-round. But conditions there may not have always been so idyllic. A new study suggests that about 4000 years ago, a prolonged drought on the island left many of the native species, such as dodo birds and giant tortoises, dead in a soup of poisonous algae and their own feces.

The die-off happened in an area known as Mare aux Songes, which once held a shallow lake that was an important source of fresh water for nonmigratory animals. Today, it’s just a grassy swamp, but beneath the surface, fossils are so common and so well preserved that the area qualifies as what scientists call a Lagerstätte, which in German means “storage space.” "What I wanted to know was, how did this drought cause this graveyard?” says Erik de Boer, a paleoecologist at the University of Amsterdam. “How did so many animals die?”

To find out, de Boer and colleagues analyzed sediment cores taken from the area. The layers in a core contain markers that can help scientists reconstruct an ecosystem’s history, such as preserved pollens and microbes. About 4200 years ago, monsoon activity declined dramatically, causing a 50-year megadrought on the island. The cores revealed that during the same time period, the ancient lake became a muddy, salty swamp. “Annually, the lake would get some fresh water in, however this drinking water turned foul during the dry season,” de Boer says.

Things got bad fairly quickly for local animals once the lake began to dry up, the team reports in the current issue of The Holocene. Sanitation appears to have become a major issue with so many animals crowding around the shrinking source of fresh water. “The animals lived around the edges, and the excrements probably got mixed up in the wetlands," de Boer says. "It’s like a big toilet.” Even worse, the researchers’ analysis shows that the feces-flooded waters encouraged the growth of single-celled algae and bacteria—diatoms and cyanobacteria—which can cause poisonous algal blooms. The circumstances combined to create what the scientists refer to as a “deadly cocktail” that they think killed many of the animals preserved as fossils at Mare aux Songes today.

Exactly how many animals died in the mucky lakebed is unclear. The study estimates that hundreds of thousands of vertebrates may be preserved at the site, but paleontologist Julian Hume, who has been studying the island for nearly 20 years, thinks that number is far too high. “They haven’t taken into account how the density of bones differs across the whole site,” Hume says. He agrees with the team’s reconstruction of the deadly cocktail, but says that most of the fossils have been found along the lake’s shores and that fewer animals died where the water was deeper.

Regardless of the total number of fatalities, researchers all agree that the megadrought didn't wipe out the island's dodos and giant tortoises completely. Both populations rebounded when the rains returned, and the animals would survive for another 3800 years before the Dutch arrived in 1638 and hunted them to extinction.