Tourists walk along the boardwalk at Grand Prismatic Spring the largest hot spring in Yellowstone National Park and third largest in the world. Grand Prismatic is about 250 by 300 feet in size, averages 160 degrees Fahrenheit and is up to 160 feet deep. The bright colors around the spring are from cyanobacteria mats. The Grand Prismatic Spring is part of the Midway Geyser Basin Excelsior Group in Yellowstone, Wyoming.