This photo provided by the National Park Service shows an unidentified man standing by an SUV-sized boulder sitting on Yellowstone National Park's West Entrance road approximately nine miles inside the park's west gate Dec. 29, 2011. The boulder apparently broke loose from a mountainside several hundred feet above the road early Thursday. Park crews have pushed the rock from the road and are assessing the slope for the potential of further slides.

Grand Prismatic Spring, Yellowstone National Park

An emergency response crew hired by Exxon Mobil cleans up an oil spill along the Yellowstone River in Laurel, Montana on July 5. An Exxon Mobil pipeline ruptured on Friday night about 150 miles downstream from Yellowstone National Park near the town of Laurel, just southwest of Billings, dumping up to 1,000 barrels, or 42,000 gallons, of crude oil into the flood-swollen river.

Oil covers a pond alongside the Yellowstone River in Laurel, Mont. on July 5.

Oil swirls in a flooded gravel pit in Lockwood, Mont., after a pipeline break early Saturday, July 2. The ExxonMobil pipeline that runs under the Yellowstone River near Billings in south-central Montana ruptured and dumped an unknown amount of oil into the waterway, prompting temporary evacuations along the river.

Tourists walk around the Grand Prismatic Spring, the largest in the United States and third largest in the world, in Yellowstone National Park, Wyoming, June 22.

A grizzly bear and her cub roam the Hayden Valley in Yellowstone National Park, Wyoming, June 24.

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.

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

View of the 'Morning Glory' hot spring with it's unique colors caused by brown, orange and yellow algae-like bacteria that thrive in the cooling water, turning the vivid aqua-blue to a murkier greenish brown, in Yellowstone National Park, Wyoming, on Thursday, June 2. Yellowstone National Park was established by the U.S. Congress and signed into law by President Grant on March 1, 1872. The park is located primarily in the U.S. state of Wyoming, though it also extends into Montana and Idaho and was the first national park in the world. It is known for its wildlife and its many geothermal features, especially the Old Faithful Geyser.

View of the 'Grand Prismatic' hot spring with it's unique colors caused by brown, orange and yellow algae-like bacteria called Thermophiles, that thrive in the cooling water turning the vivid aqua-blue to a murkier greenish brown, in Yellowstone National Park on Thursday.

Tourists walk beside a hot spring and the partially frozen Yellowstone Lake at the West Thumb Geyser Basin in Yellowstone National Park on Thursday.