This small collapsed caldera within the big Yellowstone Caldera includes the West Thumb Geyser Basin on its western shore. It extends beneath Yellowstone Lake where hot springs and even underwater geysers mix their boiling contents with the frigid lake water. A boardwalk winds through the land-based thermal features, among which are 53ft-deep, cobalt-blue Abyss Pool, and Fishing Cone Geyser.