The Grand Prismatic Spring, the largest in the United States and third largest in the world, is seen in Yellowstone National Park, Wyoming, June 22, 2011. Picture taken June 22, 2011.

The Grand Prismatic Spring, the largest in the United States and third largest in the world, is seen in Yellowstone National Park, Wyoming, June 22, 2011. Picture taken June 22, 2011.

The Grand Prismatic Spring, the largest in the United States and third largest in the world, is seen in Yellowstone National Park, Wyoming, June 22, 2011. Picture taken June 22, 2011.

A hiker walks out of the Norris Geyser Basin, one of the most earthquake active regions of the park, in Yellowstone National Park, Wyoming, June 21, 2011.

The nearly constantly erupting Clepsydra Geyser in the Fountain Paint Pot area in Yellowstone National Park, Wyoming, June 21, 2011.

People stand at the brink of the 93-meter tall Yellowstone River Lower Falls in Yellowstone National Park, Wyoming, June 24, 2011. Picture taken June 24, 2011.

A cinnamon black bear, a subspecies of the American black bear, runs between cars near Madison in Yellowstone National Park, Wyoming, June 19, 2011. Picture taken June 19, 2011.

Grizzly bear cubs play on the snow in the Hayden Valley in Yellowstone National Park, Wyoming, June 24, 2011. Picture taken June 24, 2011.

A wolf walks with a set of elk legs in the Hayden Valley after killing and eating it in Yellowstone National Park, Wyoming, June 20, 2011. Elks are the most common food of the wolves in the park. Picture taken June 20, 2011.