Quick answer
Calistoga is a spa town at the north end of Napa Valley, founded in the 1860s by Sam Brannan, California's first millionaire, who named it after a slip of the tongue. It runs on geothermal hot springs and volcanic-ash mud baths. Its Old Faithful geyser is actually a well drilled for water in the late nineteenth century that still erupts on schedule.
Full transcript
Saratoga, Slightly Rearranged
Sam Brannan was California's first millionaire, a gold-rush merchant who also published San Francisco's first English-language newspaper. He bought more than two thousand acres around the hot springs at the head of the Napa Valley and set out to build a spa like Saratoga, New York. By his own account, the name came from a slip of the tongue that turned the Saratoga of California into the Calistoga of Sarafornia. The name stuck in 1867, his resort had opened in 1862, and the railroad arrived in 1868, carrying ferry passengers up from San Francisco.
Water, Steam, and Mud
Everything here runs on the same geothermal heat. The springs feed the spas and the famous mud baths — you sit in warm volcanic ash, which is exactly as odd as it sounds. In 1920 a candy-store owner drilling for cold water tapped a hot spring instead, and in 1924 he was bottling Calistoga mineral water. The town's Old Faithful is not quite what it pretends to be: it erupts from the casing of a well that someone drilled in the late eighteen hundreds looking for water, opening up what a geyser expert called a dead geyser. It still erupts, and nobody has to believe in magic to watch it.
What to Do
Head for the geyser first and be ready — it erupts without warning and does not wait for you. Then take the local treatment properly: a soak in a hot spring, a mud bath if you are brave, and a bottle of the town's own sparkling mineral water. The writer Robert Louis Stevenson honeymooned near here and turned the trip into a book, The Silverado Squatters. Walk up the old mine road toward Mount Saint Helena, and picture the valley below him — the town still steaming, the way it does today.