The commanding features that initially attracted interest, and led to the preservation of Yellowstone as a national park, were geological: the geothermal phenomena (there are more geysers and hot springs here than in the rest of the world combined), the colorful Grand Canyon of the Yellowstone River, fossil forests, and the size and elevation of Yellowstone Lake.

Geothermal activity results when surface water seeps down to meet the earth's molten rock and in Yellowstone, this rock is only three miles below the surface.

Hot Springs. The color, ranging from blue, to orange, to yellow, to black, in thermal pools is caused by the water's temperature, algae, minerals, reflection and absorption of light by particles in the water. The largest hot spring in Yellowstone is Grand Prismatic Spring, which is 370 feet in diameter and is accessed by the Midway Geyser Basin boardwalk. Another favorite is the Morning Glory Pool.

Geysers. Geysers are hot springs which periodically erupt as pressure mounts deep under the surface. Some geysers erupt every minute, some remain inactive for years. The park contains 150 to 200 geysers, including Old Faithful which erupts on average every 79 minutes.

Mud Pots are another form of hot spring. They form as hot water mixes with clay and minerals. Mud pots can be very colorful, depending on the mineral content and are sometimes called "paint pots."

Fumaroles. When water vapor and gases shoot from underground they create a "dry geyser." Often the expulsion is so violent that the ground shakes and causes a sound like thunder.