About 66 million years ago, the Cretaceous ended in a mass extinction that eliminated all non-avian dinosaurs along with ammonites and many marine plankton groups. Roughly three-quarters of species are estimated to have disappeared.

The strongest smoking gun is the Chicxulub asteroid impact beneath today's Yucatán Peninsula. Evidence includes a global iridium-rich boundary layer, shocked minerals, impact ejecta, and a crater about 180 km across.

The NASA/JPL image above is not fantasy impact art. It is a computer-enhanced shaded-relief/topographic view of the modern Yucatán Peninsula. The subtle semicircular trough marks the crater's outer boundary; the crater itself is mostly buried under later limestone, and the height/color exaggeration makes a very low-relief feature visible.

The killer was not simply the blast near Mexico. Dust, sulfate aerosols, soot, and ejecta spread globally, sharply reducing sunlight and photosynthesis. The resulting impact winter collapsed food webs on land and in the oceans.

Large eruptions in India's Deccan Traps were also occurring around this time and affected climate. Their exact contribution to the extinction remains debated, but the asteroid is the clearest abrupt trigger.

One useful correction: dinosaurs did not vanish completely. Birds are living dinosaurs.