(See "'Sleeping' Volcanoes Can Wake Up Faster Than Thought.")

A mile and a half wide (2.4 kilometers) and 600 feet (180 meters) deep, California's Ubehebe Crater came explosively into being long ago when rising magma hit water. (Video: Volcanoes 101.)

The bomblike steam eruption produced a mushroom cloud that, as it collapsed, sent rocky debris flowing out sideways at 200 miles an hour (320 kilometers an hour) to a distance of a few kilometers, according to a geologic analysis of rock deposits at the site, study co-author Brent Geohring said.

The question is, when?

Scientists had assumed the explosion occurred comfortably in the past, most likely several thousand years ago, when the Death Valley area was wetter.

The explosive mixing of