Grand Prismatic Spring Yellowstone's largest hot spring, at 370 feet in diameter, is accessed by taking the Midway Geyser Basin boardwalk. This pool gets its name because of its resemblance to the morning glory flower. It is no longer as deep blue as it once was, because its vent is becoming clogged by debris visitors have thrown into it. This body of water even creates its own weather, forming cumulus clouds during the day that often shower the area by evening. The lake sits in a large caldera, or crater, that was formed by a volcano and then carved and filled by glaciers some 12,000 years ago.
Streams flowing into the lake provide it and the Yellowstone River with abundant fish.
West Thumb Geyser Basin Located on Yellowstone Lake's shore, this small thermal area holds lakeshore geysers, hot springs, and bubbling paint pots. Fishing Cone is one of many lakeshore geysers submerged here until the lake's water level drops in late summer. Fishing Cone gets its name from the fact that fishermen used to catch trout from the lake and then promptly drop them into its boiling waters to cook.