In September 2015, scientists witnessed something never seen before: two black holes colliding. Both about 30 times as big as our Sun, they had been orbiting each other for millions of years. A fraction of a second before the crash, they sent a vibration across the universe at the speed of light that was picked up by the LIGO detector. So what are these ripples in space? Amber L. Stuver explains. Lesson by Amber L. Stuver, animation by Eoin Duffy.