Inch by inch, the team pulled up the skinny core of ghostly white limestone from the ocean floor, gazing at the compressed remains of ancient organisms that died tens of millions of years ago. But then a stark divide appeared as the layers abruptly darkened.

“It was nothing like the stuff above,” recalls Sean Gulick, a co-chief scientist of the expedition and a researcher at the University of Texas at Austin.

This change in the rock marks one of the most catastrophic events in Earth’s history, some 66 million years ago, when an epic asteroid slammed into the sea just offshore of Mexico’s Yucatán Peninsula. The impact triggered a nightmarish sequence of events that sent some 75 percent of plant and