Knoll noted that the world can change a lot without producing huge losses; ice ages, for instance, come and go. “What the geological record tells us is that it’s time to worry when the rate of change is fast,” he told me. In the case of the end-Permian extinction, Knoll and many other researchers believe that the trigger was a sudden burst of volcanic activity; a plume of hot mantle rock from deep in the earth sent nearly a million cubic miles’ worth of flood basalts streaming over what is now Siberia. The eruption released enormous quantities of carbon dioxide, which presumably led—then as now—to global warming, and to significant changes in ocean chemistry.

“CO 2 is a paleontologist’s dream,” Knoll told me. “It can kill things directly, by physiological effects, of which ocean acidification is the best known, and it can kill things by changing the climate. If it gets warmer faster than you can migrate, you’re in trouble.”

In the end, the most deadly aspect of human activity may simply be the pace of it. Just in the past century, CO 2 levels in the atmosphere have changed by as much—a hundred parts per million—as they normally do in a hundred-thousand-year glacial cycle. Meanwhile, the drop in ocean pH levels that has occurred over the past fifty years may well exceed anything that happened in the seas during the previous fifty million. In a single afternoon, a pathogen like Bd can move, via United or American Airlines, halfway around the world. Before man entered the picture, such a migration would have required hundreds, if not thousands, of years—if, indeed, it could have been completed at all.

Currently, a third of all amphibian species, nearly a third of reef-building corals, a quarter of all mammals, and an eighth of all birds are classified as “threatened with extinction.” These estimates do not include the species that humans have already wiped out or the species for which there are insufficient data. Nor do the figures take into account the projected effects of global warming or ocean acidification. Nor, of course, can they anticipate the kinds of sudden, terrible collapses that are becoming almost routine.

I asked Knoll to compare the current situation with past extinction events. He told me that he didn’t want to exaggerate recent losses, or to suggest that an extinction on the order of the end-Cretaceous or end-Permian was imminent. At the same time, he noted, when the asteroid hit the Yucatán “it was one terrible afternoon.” He went on, “But it was a short-term event, and then things started getting better. Today, it’s not like you have a stress and the stress is relieved and recovery starts. It gets bad and then it keeps being bad, because the stress doesn’t go away. Because the stress is us.”

Aeolus Cave, in Dorset, Vermont, is believed to be the largest bat hibernaculum in New England; it is estimated that, before white nose hit, more than two hundred thousand bats—some from as far away as Ontario and Rhode Island—came to spend the winter there.

In late February, I went with Hicks to visit Aeolus. In the parking lot of the local general store, we met up with officials from the Vermont Fish and Wildlife Department, who had organized the trip. The entrance to Aeolus is about a mile and a half from the nearest road, up a steep, wooded hillside. This time, we approached by snowmobile. The temperature outside was about twenty-five degrees—far too low for bats to be active—but when we got near the entrance we could, once again, see bats fluttering around. The most senior of the Vermont officials, Scott Darling, announced that we’d all have to put on latex gloves and Tyvek suits before proceeding. At first, this seemed to me to be paranoid; soon, however, I came to see the sense of it.

Aeolus is a marble cave that was created by water flow over the course of thousands of years. The entrance is a large, nearly horizontal tunnel at the bottom of a small hollow. To keep people out, the Nature Conservancy, which owns the cave, has blocked off the opening with huge iron slats, so that it looks like the gate of a medieval fortress. With a key, one of the slats can be removed; this creates a narrow gap that can be crawled (or slithered) through. Despite the cold, there was an awful smell emanating from the cave—half game farm, half garbage dump. When it was my turn, I squeezed through the gap and immediately slid on the ice, into a pile of dead bats. The scene, in the dimness, was horrific. There were giant icicles hanging from the ceiling, and from the floor large knobs of ice rose up, like polyps. The ground was covered with dead bats; some of the ice knobs, I noticed, had bats frozen into them. There were torpid bats roosting on the ceiling, and also wide-awake ones, which would take off and fly by or, sometimes, right into us.

Why bat corpses pile up in some places, while in others they get eaten or in some other way disappear, is unclear. Hicks speculated that the weather conditions at Aeolus were so harsh that the bats didn’t even make it out of the cave before dropping dead. He and Darling had planned to do a count of the bats in the first chamber of the cave, known as Guano Hall, but this plan was soon abandoned, and it was decided just to collect specimens. Darling explained that the specimens would be going to the American Museum of Natural History, so that there would at least be a record of the bats that had once lived in Aeolus. “This may be one of the last opportunities,” he said. In contrast to a mine, which has been around at most for a few centuries, Aeolus, he pointed out, has existed for millennia. It’s likely that bats have been hibernating there, generation after generation, since the end of the last ice age.

“That’s what makes this so dramatic—it’s breaking the evolutionary chain,” Darling said.

He and Hicks began picking dead bats off the ground. Those which were too badly decomposed were tossed back; those which were more or less intact were sexed and placed in two-quart plastic bags. I helped out by holding open the bag for females. Soon, it was full and another one was started. It struck me, as I stood there holding a bag filled with several dozen stiff, almost weightless bats, that I was watching mass extinction in action.

Several more bags were collected. When the specimen count hit somewhere around five hundred, Darling decided that it was time to go. Hicks hung back, saying that he wanted to take some pictures. In the hours we had been slipping around the cave, the carnage had grown even more grotesque; many of the dead bats had been crushed and now there was blood oozing out of them. As I made my way up toward the entrance, Hicks called after me: “Don’t step on any dead bats.” It took me a moment to realize that he was joking. ♦