The blue water spring is about 13 feet in diameter and 1 foot below the rim, USGS said. The water in the pool was measured at 109°F.

Where previously there had been a rather featureless patch of ground at the northwestern tip of a landmark known informally as 'Tree Island', there was now a gaping hole about 4 meters (13 feet) wide,

The dominant rock type in the Yellowstone region is rhyolite, and that is very rich in silica. As hot water circulates underground, it can dissolve that silica and bring it to the surface, where it is deposited as the water cools.