There's a lake that's made entirely of bat guano - Apparently.
In the Danakil Depression of Ethiopia, Lake Natron is a saltwater lake that's often covered in a thick layer of calcium carbonate, which is formed from the remains of millions of bats that fall from the sky and die in the lake's waters. The lake's unique chemistry, combined with the high salt concentration, creates a self-sustaining ecosystem that's home to only a few hardy microorganisms. This natural phenomenon has been extensively studied by scientists and photographed by adventurers.