The thermal pools in Yellowstone National Park are known for the beauty. But they have a dangerous side also. Surrounding the pools is a thin crust of bacteria known collectively as thermophiles. These organisms can survive temperatures that approach the boiling point. For visitors to the Park, the fragile surface created by the colorful bacteria can be deadly. There is no real support for these formations and stepping off the wooden walkways can plunge the unwary into the boiling water.