Sure, they come to see the geysers; yet not many visitors to Yellowstone seem to realize they are wandering through the caldera of one of the largest active volcanoes in the world. The evidence is all around, jarring in its juxtaposition to the other wonders of the park: malodorous sulphur vents near fragrant pine forests, boiling hot springs in the middle of crystal-blue pools, desolate parched dead zones bordering verdant grasslands.
Especially interesting to me were the vivid colors of many of the hydrothermal features. The bright reds, yellows, and greens found in and near these scalding waters are, surprisingly, caused by thermophilic organisms -- heat-loving bacteria which grow in conditions hostile to all other forms of life.
Old Faithful is the best-known geyser in the park, probably because it erupts the most frequently of the larger geysers; but it is neither the largest nor the most regular. The park rangers "predict" the eruptions of six major geysers each day on a board in the Old Faithful visitor center, so you can drop in anytime and head for the next likely performer, much like catching the next feature at the local multiplex.
We happened by one afternoon when Grand, the world's tallest predictable geyser, was on the board in a half-hour, so we hurried over to watch. However, Grand hadn't seen the day's schedule and chose to keep us (and several hundred others) waiting another hour and 45 minutes. I can't complain too much; it was worth the wait for about ten minutes of exuberant nonstop spouting. There's something both fascinating and comforting about natural events that man cannot control.