This activity could have a whole range of consequences. In a study released last year, the United States Geological Survey (USGS) said possible hazards could include hydrothermal explosions, when steam breaks through the surface and forms a crater. That has happened 26 times in the park's 127 years of record-keeping. The USGS discounted chances for cataclysmic eruption of the caldera, noting that the hot, active magma chamber below Yellowstone has turned into "largely crystallized mush." But the same study also said: "Depending on the nature and magnitude of a particular hazardous event and the particular time and season when it might occur, 70,000 to more than 100,000 persons could be affected; the most violent events could affect a broader region or even continent-wide areas." Jake Lowenstern, Ph.D., YVO's chief scientist, who also is part of the USGS Volcano Hazards Team, told TIME that a supervolcano event does not appear to be imminent. "We don't think the amount of magma exists that would create one of these large eruptions of the past," he said.

"It is still possible to have a volcanic eruption comparable to other volcanoes. But we would expect to see more and larger quakes, deformation and precursory explosions out of the lake. We don't believe that anything strange is happening right now." Last summer YVO installed new instrumentation in boreholes 500 to 600 ft. deep to better detect ground deformation.?Says Lowenstern: "We have a lot more ability to look at all the data now." (See an interactive graphic depicting how scientists monitor volcanoes.)

The Yellowstone Caldera — formed by the massive upheaval 642,000 years ago that spread airborne debris all the way to the Gulf of Mexico — is nowhere close to being extinct. Areas of the park's topography inflate like a bellows because of magma infusing into volcanic chambers about 6 miles below the surface. About 1,000 to 2,000 tremors a year (mostly small) have been recorded since 2004, when interpretation of satellite imagery with GPS readings indicated the caldera had been rising as much as 3 in. a year. The past week's number of tremors — about 400 — is considered unusual.
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