How did the dinosaurs go extinct?

The leading, strongly supported explanation is catastrophic impact. About 66 million years ago, an asteroid or comet roughly six miles wide slammed into what is now the Yucatan Peninsula in Mexico, leaving the enormous Chicxulub crater that scientists have since located and dated. The impact released energy dwarfing every nuclear weapon combined, igniting wildfires, launching debris worldwide, and, most lethally, hurling so much dust and soot into the atmosphere that it blocked sunlight for months to years. That impact winter collapsed plant life and the food chains built on it, killing an estimated three quarters of all species on Earth, including all the large dinosaurs.

The evidence is compelling: a worldwide layer of sediment from that exact moment is rich in iridium, an element rare on Earth but common in asteroids, plus shocked quartz and tiny glass beads formed by the blast, all pointing to a single global event. Some scientists argue that massive volcanic eruptions in India (the Deccan Traps) were already stressing the planet's climate and may have contributed, and the debate over how much each factor mattered continues, but the asteroid is the consensus primary cause. The crucial twist most people miss: the dinosaurs did not all die. Birds are living dinosaurs, direct descendants of small feathered theropods that survived the extinction, which is why a sparrow at your feeder is, quite literally, a dinosaur.