The Castle Geyser in the Upper Geyser Basin, Yellowstone National Park, Montana, USA. The oldest national park in the world, Yellowstone attracts thousands of tourists and is so heavily overused that the authorities may decide to restrict the number of visitors.

Castle Geyser, in Yellowstone National Park, Wyoming, USA. Castle Geyser usually erupts every 9-11 hours. The 20-minute water phase of a major eruption is followed by a 40-minute steam phase. Castle Geyser often has minor eruptions which last only a few minutes, after which it becomes unpredictable until it has its next major eruption.