66 million years ago, an object somewhere between the size of Mt. Everest and the country of Luxembourg (or the island of Puerto Rico) slammed into what would become the Yucatan peninsula in Mexico at a speed of 20 kilometers per second, or 72,000 km/h. The impact that an asteroid of that size moving at that speed made was unimaginably vast: scientists estimate that the energy released was around 100 million times that produced by Tsar Bomba, the most powerful hydrogen bomb ever built. The devastation that impact caused, helped along by wide-scale volcanic eruptions and climate change, killed a large percentage of life on earth, wiping out virtually all land and amphibian species larger than 25kg in body weight.
It could happen again. Objects from outer space hit the earth with alarming regularity. 50,000 years ago, a nickel-iron meteorite 50 meters across struck Arizona, creating the aptly named Meteor Crater. In 1908, a slightly larger object exploded a few kilometers above the forests of Siberia, near Tunguska, flattening 80 million trees. And in 2013, a 20 meter object lit up the skies above Chelyabinsk in Russia, eventually detonating some 30 kilometers up. The ensuing explosion and shock wave destroyed windows and damaged buildings in an area a hundred kilometers long and tens of kilometers in length.