The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named. It was formed by a large asteroid or comet about 10 kilometres (6 miles) in diameter, the Chicxulub impactor, striking the Earth. The date of the impact coincides precisely with the Cretaceous–Paleogene boundary (K–Pg boundary), slightly less than 66 million years ago, and a widely accepted theory is that worldwide climate disruption from the event was the cause of the Cretaceous–Paleogene extinction event, a mass extinction in which 75% of plant and animal species on Earth suddenly became extinct, including all dinosaurs and all enantiornithine birds, also known as "opposite birds" because their leg bones fuse in the opposite direction of modern birds. The enantiornithine birds were the dominant land birds of the Mesozoic. The birds that survived were ornithurine birds, or modern birds, which were shorebirds during the Mesozoic, and they are the ancestors of all known living birds. The crater is more than 180 kilometers (110 miles) in diameter and 20 km (12 mi) in depth, well into the continental crust of the region of about 10–30 km depth. It makes the feature the third of the largest confirmed impact structures on Earth.
The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with Alan Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.
Explanation from: https://en.wikipedia.org/wiki/Chicxulub_crater