Mist from the wide Excelsior Geyser envelops visitors who cross the footbridge and climb past a multicolored bank of the Firehole River. Runoff from acidic Grand Prismatic Spring has created terraced algae mats, often decorated with hoof prints from bison. At 370ft across and 120ft deep, Grand Prismatic is the second-largest hot spring in the world. Its rainbow colors were created by dissolved iron and other minerals, manifesting in red, orange and black hues.