Just yesterday, in geological time, the area that is now Yellowstone National Park exploded, creating a caldera 35 miles wide and 45 miles long. A mere 640,000 years later, geysers, mud pots, and fumeroles remind visitors that this precious ecosystem remains perched atop a vast hydrothermal underworld. The surface subsides or bulges by several inches annually due to deep migrations of molten rock.

Entrances open 24 hours per day during main season (Memorial Day weekend through October).

Winter access: The road between Gardiner, MT and Cooke City, MT is the only road plowed year round for vehicle access. During the winter season, no other roads are plowed and access is possible only via guided snowmobile or snow-coach tours. Summer access: plows typically open roads on the west side of the park by mid-April, then gradually the remaining roads are plowed until everything is generally open by Memorial Day weekend. The park closes for the season the first Monday in November. Ranger-led activities and programs are offered during the summer and winter seasons. The park's official web site (www.nps.gov/yell) provides a wealth of trip planning assistance and extensive information on Yellowstone's natural and cultural resources.

Fees: Yes - contact park or visit website