Deer Cave, Borneo (aka pile of bat dung)This cave was featured on the BBC series Planet Earth, so I had to check it out while in Borneo. You have to get to Mulu National Park and stay in the dorms offered by the park. From there you can take guided tours to the caves and see the ribbon of bats fly out at dusk in search of food. They estimate 2.5-3.5 million bats live in this cave! So naturally, this cave is FULL of bat dung. Cave residents also include millions of roaches feeding off of the bats and sparrows.