“So you like it?” I say.

“Oh, no, I love it,” he answers with profound sincerity. “I mean I really love it here. The winters are tough and the pay’s not too hot, but when it’s good, it’s just-”

He interrupted himself to point out a distant gap in a range of mountains to the west, which had just come into view over a rise. The mountains, he told me, were known as the Gallatins. “That gap is sixty or maybe seventy miles across. For a long time nobody could understand why that gap was there, and then Bob Christiansen realized that it had to be because the mountains were just blown away. When you’ve got sixty miles of mountains just obliterated, you know you’re dealing with something pretty potent. It took Christiansen six years to figure it all out.”

I asked him what caused Yellowstone to blow when it did.

“Don’t know. Nobody knows. Volcanoes are strange things. We really don’t understand them at all. Vesuvius, in Italy, was active for three hundred years until an eruption in 1944 and then it just stopped. It’s been silent ever since. Some volcanologists think that it is recharging in a big way, which is a little worrying because two million people live on or around it. But nobody knows.”

“And how much warning would you get if Yellowstone was going to go?”

He shrugged. “Nobody was around the last time it blew, so nobody knows what the warning signs are. Probably you would have swarms of earthquakes and some surface uplift and possibly some changes in the patterns of behavior of the geysers and steam vents, but nobody really knows.”

“So it could just blow without warning?”

He nodded thoughtfully. The trouble, he explained, is that nearly all the things that would constitute warning signs already exist in some measure at Yellowstone. “Earthquakes are generally a precursor of volcanic eruptions, but the park already has lots of earthquakes-1,260 of them last year. Most of them are too small to be felt, but they are earthquakes nonetheless.”

A change in the pattern of geyser eruptions might also be taken as a clue, he said, but these too vary unpredictably. Once the most famous geyser in the park was Excelsior Geyser. It used to erupt regularly and spectacularly to heights of three hundred feet, but in 1888 it just stopped. Then in 1985 it erupted again, though only to a height of eighty feet. Steamboat Geyser is the biggest geyser in the world when it blows, shooting water four hundred feet into the air, but the intervals between its eruptions have ranged from as little as four days to almost fifty years. “If it blew today and again next week, that wouldn’t tell us anything at all about what it might do the following week or the week after or twenty years from now,” Doss says. “The whole park is so volatile that it’s essentially impossible to draw conclusions from almost anything that happens.”

Evacuating Yellowstone would never be easy. The park gets some three million visitors a year, mostly in the three peak months of summer. The park’s roads are comparatively few and they are kept intentionally narrow, partly to slow traffic, partly to preserve an air of picturesqueness, and partly because of topographical constraints. At the height of summer, it can easily take half a day to cross the park and hours to get anywhere within it. “Whenever people see animals, they just stop, wherever they are,” Doss says. “We get bear jams. We get bison jams. We get wolf jams.”

In the autumn of 2000, representatives from the U.S. Geological Survey and National Park Service, along with some academics, met and formed something called the Yellowstone Volcanic Observatory. Four such bodies were in existence already-in Hawaii, California, Alaska, and Washington-but oddly none in the largest volcanic zone in the world. The YVO is not actually a thing, but more an idea-an agreement to coordinate efforts at studying and analyzing the park’s diverse geology. One of their first tasks, Doss told me, was to draw up an “earthquake and volcano hazards plan”-a plan of action in the event of a crisis.

“There isn’t one already?” I said.

“No. Afraid not. But there will be soon.”

“Isn’t that just a little tardy?”

He smiled. “Well, let’s just say that it’s not any too soon.”

Once it is in place, the idea is that three people-Christiansen in Menlo Park, California, Professor Robert B. Smith at the University of Utah, and Doss in the park-would assess the degree of danger of any potential cataclysm and advise the park superintendent. The superintendent would take the decision whether to evacuate the park. As for surrounding areas, there are no plans. If Yellowstone were going to blow in a really big way, you would be on your own once you left the park gates.

Of course it may be tens of thousands of years before that day comes. Doss thinks such a day may not come at all. “Just because there was a pattern in the past doesn’t mean that it still holds true,” he says. “There is some evidence to suggest that the pattern may be a series of catastrophic explosions, then a long period of quiet. We may be in that now. The evidence now is that most of the magma chamber is cooling and crystallizing. It is releasing its volatiles; you need to trap volatiles for an explosive eruption.”

In the meantime there are plenty of other dangers in and around Yellowstone, as was made devastatingly evident on the night of August 17, 1959, at a place called Hebgen Lake just outside the park. At twenty minutes to midnight on that date, Hebgen Lake suffered a catastrophic quake. It was magnitude 7.5, not vast as earthquakes go, but so abrupt and wrenching that it collapsed an entire mountainside. It was the height of the summer season, though fortunately not so many people went to Yellowstone in those days as now. Eighty million tons of rock, moving at more than one hundred miles an hour, just fell off the mountain, traveling with such force and momentum that the leading edge of the landslide ran four hundred feet up a mountain on the other side of the valley. Along its path lay part of the Rock Creek Campground. Twenty-eight campers were killed, nineteen of them buried too deep ever to be found again. The devastation was swift but heartbreakingly fickle. Three brothers, sleeping in one tent, were spared. Their parents, sleeping in another tent beside them, were swept away and never seen again.

“A big earthquake-and I mean big-will happen sometime,” Doss told me. “You can count on that. This is a big fault zone for earthquakes.”

Despite the Hebgen Lake quake and the other known risks, Yellowstone didn’t get permanent seismometers until the 1970s.

If you needed a way to appreciate the grandeur and inexorable nature of geologic processes, you could do worse than to consider the Tetons, the sumptuously jagged range that stands just to the south of Yellowstone National Park. Nine million years ago, the Tetons didn’t exist. The land around Jackson Hole was just a high grassy plain. But then a forty-mile-long fault opened within the Earth, and since then, about once every nine hundred years, the Tetons experience a really big earthquake, enough to jerk them another six feet higher. It is these repeated jerks over eons that have raised them to their present majestic heights of seven thousand feet.

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?”