Grand Prismatic Spring
Grand Prismatic Spring in Yellowstone National Park. It is 250 by 300 feet across and 160 feet deep. Note the people on the boardwalk for scale.
This is the largest hot spring in Yellowstone and third largest in the world. The discharge is about 560 gallons of water per minute. The color is from thermophilic bacteria that live in the different temperature gradients.
Swimming is not allowed because it would result in almost certain death. The water is between 160-188°F which is near boiling hot.