Large

Grotto Geyser

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One of the most unusual geysers in Yellowstone (and that's saying something!) is marked by large, irregularly formed white rocks pockmarked with holes. It also erupts quite irregularly - sometimes staying quiet for 8 to 10 hours. The longer the eruption, the longer the interval. Some eruptions have lasted for 1 to 8 hours, although one was recorded to last for a little over a day! The abnormal, white shape around the geyser was formed from sinter which is a mineral deposited by the water shooting out of the geyser.