In 1980, the physicist Luís Alvarez and his son Walter advanced a startling theory about the demise of the dinosaurs: that it was caused by forces that came from beyond this world. They hypothesized that perhaps a meteor impact had ended the age of the dinosaurs. The primary evidence was that in soil core samples taken in locations around the globe, iridium, a substance very rare on Earth but prevalent on asteroids, had been found in a thin layer of clay separating the fossil-rich rock of the late Cretaceous period (the end of the dinosaur age) and the sparsely fossiled rock of the Tertiary period that followed. The Alvarezes hypothesized that a very large extraterrestrial object had slammed into the planet, sending an enormous fireball into the stratosphere, along with vast amounts of debris. A great cloud of dust enshrouded Earth, blocking sunlight for months, even years, and plants and animals perished in the ensuing cold and dark. When the dust finally settled back to Earth, it formed the telltale worldwide layer of iridium in the clay. The scientific world was not impressed by the theory. Indeed, some scientists scoffed at the Alvarezes’ hypothesis, but in 1990 scientists realized that a crater of 112 miles in diametre in Mexico and dated at 65 million years old might be evidence that the dinosaurs had indeed died out due to the effects of a giant meteor.