"The article called the site a ‘Pompeii of prehistoric animals,' " he told me, "which was unfortunate because just afterward we realized that the animals hadn't died suddenly at all. They were all suffering from something called hypertrophic pulmonary osteodystrophy, which is what you would get if you were breathing a lot of abrasive ash—and they must have been breathing a lot of it because the ash was feet thick for hundreds of miles." He picked up a chunk of grayish, claylike dirt and crumbled it into my hand. It was powdery but slightly gritty. "Nasty stuff to have to breathe," he went on, "because it's very fine but also quite sharp. So anyway they came here to this watering hole, presumably seeking relief, and died in some misery. The ash would have ruined everything. It would have buried all the grass and coated every leaf and turned the water into an undrinkable gray sludge. It couldn't have been very agreeable at all."

The BBC documentary had suggested that the existence of so much ash in Nebraska was a surprise. In fact, Nebraska's huge ash deposits had been known about for a long time. For almost a century they had been mined to make household cleaning powders like Comet and Ajax. But curiously no one had ever thought to wonder where all the ash came from.