The ancient Romans had it right, as did the nostalgic Japanese poet Basho, and all of those Victorian ladies fighting off "hysteria." When things get cold, stressful or unbearably dismal, immersing yourself in hot water emanating from a volcanic crack in the ground is as close as you can come to calling it a day and climbing back into the womb. If it doesn't cure what ails you, at the very least, you'll cease to worry about it for a bit.
In the days before water heaters and indoor plumbing, the presence of thermal water could make or break a town -- and many of these springs have been travel hot spots for millennia. You can don plush robes at swanked-out spas so decadent that Nero would feel right at home; hit a historic 19th-century bathhouse for a more medicinal "taking of the waters"; head deep into the mountains to a Japanese Zen forest retreat; or float unencumbered (and perhaps naked) in a private hot-water rock garden on the edge of a river.