That nine hundred years is an average—and a somewhat misleading one. According to Robert B. Smith and Lee J. Siegel in Windows into the Earth, a geological history of the region, the last major Teton quake was somewhere between about five and seven thousand years ago. The Tetons, in short, are about the most overdue earthquake zone on the planet.
Hydrothermal explosions are also a significant risk. They can happen anytime, pretty much anywhere, and without any predictability. "You know, by design we funnel visitors into thermal basins," Doss told me after we had watched Old Faithful blow. "It's what they come to see. Did you know there are more geysers and hot springs at Yellowstone than in all the rest of the world combined?"
I didn't know that.
He nodded. "Ten thousand of them, and nobody knows when a new vent might open."
We drove to a place called Duck Lake, a body of water a couple of hundred yards across. "It looks completely innocuous," he said. "It's just a big pond. But this big hole didn't used to be here. At some time in the last fifteen thousand years this blew in a really big way. You'd have had several tens of millions of tons of earth and rock and superheated water blowing out at hypersonic speeds. You can imagine what it would be like if this happened under, say, the parking lot at Old Faithful or one of the visitors' centers." He made an unhappy face.