Incredible geothermal vents in southwest Bolivia, replete with bubbling mud-like substances and crackling sounds rumbling from below. This was actually an unexpectedly scary stop on our tour - the smoke is flowing out of holes easily big enough for a person to fall into, which would be a rather scary thing to have happen. Logically, I know you wouldn't tumble all the way to the center of the earth. But looking into a treacherous hole that you can't see the bottom of, especially one that's billowing smoke and groaning ominously, is....disconcerting.
Interesting note: though often described as such, these are not geysers, the term "geyser" is actually one of the most misused in all of geology. Geysers involve discharges of water (as opposed to just steam) and exist in only 5 places on earth. ((Kamchatka, Russia; Taupo, New Zealand; Haukadalar, Iceland; El Tatio, Chile; Yellowstone, USA))