Tiny flecks of white dot a section of crumbly rocks along Texas's Brazos River. To a casual observer, the grains may seem like unremarkable bits of sand, but within their oddball shapes lie clues to the most catastrophic day in our planet's history.

Some 66 million years ago, a six-mile wide asteroid slammed into the ocean off the coast of Mexico’s Yucatán Peninsula, carving out a 110-mile wide crater known as Chicxulub. In an instant, the trajectory of life on Earth was forever changed. The impact kicked off wildfires and tsunamis across thousands of miles. Then swings in global climate—a dramatic period of cooling followed by a long period of warming—ushered in the extinction of some 75