Yellowstone is an active volcano. Surface features such as geysers and hot springs are direct results of the region's underlying volcanism.
This picture shows the Yellowstone Caldera, where the hotpsot of molten lava is only about a mile below the earths surface. This picture shows from the north Yellowstone Lake, in the foreground, right in the center of the caldera, Lewis and Shoshone Lakes just beyond, and the Grand Tetons in the distance to the south.
Yellowstone is an active supervolcano which when it erupts, will cover much of the continental United States in a blanket of ash . That ash will clog the atmosphere enough to block out the sun, disrupting the global climate enough to cause mass extinctions.
It could be tens of thousands of years before the next Yellowstone eruption. The last full-scale eruption of this kind occurred 640,000 years ago, and the ones prior to that occurred 1.3 million years and 2.1 million years ago. Interspersed with the big ones have been smaller-scale but still major eruptions, most recently 70,000 years ago.
Kayaking on Yellowstone Lake is an excellent way to experience how alive our planet really is.