Sixty-six million years ago, an asteroid impact seems to have nearly eliminated 99.9 percent of life on Earth, but scientists still have questions about the conditions leading up to this mass extinction. A dark layer of ash and debris known as the KT boundary marks the dividing line: above it is the Tertiary Period, below it is the Cretaceous period, or the time of dinosaurs. Until 2013, scientists had found very few dinosaur remains in the nine feet of material immediately below the boundary, and there was no agreement about the dinosaurs’ decline leading up to the asteroid impact. Then a paleontology student named Robert DePalma made a monumental discovery in North Dakota.

For The New Yorker, Douglas Preston tells DePalma’s and the asteroid’s story, as he spends time with him at the secretive site of what might be the greatest scientific discovery of the century. As DePalma is not yet a known entity and his discovery is just coming to light, some people in the scientific community find him unreliable and doubt his interpretation of the fossil evidence.

The following day, DePalma noticed a small disturbance preserved in the sediment. About three inches in diameter, it appeared to be a crater formed by an object that had fallen from the sky and plunked down in mud. Similar formations, caused by hailstones hitting a muddy surface, had been found before in the fossil record. As DePalma shaved back the layers to make a cross-­section of the crater, he found the thing itself—not a hailstone but a small white sphere—at the bottom of the crater. It was a tektite, about three millimetres in diameter—the fallout from an ancient asteroid impact. As he continued excavating, he found another crater with a tektite at the bottom, and another, and another. Glass turns to clay over millions of years, and these tektites were now clay, but some still had glassy cores. The microtektites he had found earlier might have been carried there by water, but these had been trapped where they fell—on what, DePalma believed, must have been the very day of the disaster.

“When I saw that, I knew this wasn’t just any flood deposit,” DePalma said. “We weren’t just near the KT boundary—this whole site is the KT boundary!” From surveying and mapping the layers, DePalma hypothesized that a massive inland surge of water flooded a river valley and filled the low-lying area where we now stood, perhaps as a result of the KT-impact tsunami, which had roared across the proto-Gulf and up the Western Interior Seaway. As the water slowed and became slack, it deposited everything that had been caught up in its travels—the heaviest material first, up to whatever was floating on the surface. All of it was quickly entombed and preserved in the muck: dying and dead creatures, both marine and freshwater; plants, seeds, tree trunks, roots, cones, pine needles, flowers, and pollen; shells, bones, teeth, and eggs; tektites, shocked minerals, tiny diamonds, iridium-laden dust, ash, charcoal, and amber-smeared wood. As the sediments settled, blobs of glass rained into the mud, the largest first, then finer and finer bits, until grains sifted down like snow.

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