Detected for the very first time in 2015, gravitational waves are ripples in the fabric of spacetime that occur when massive objects move around. The first waves were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) four days before it was scheduled to officially begin scientific observations. The signal came from two black holes more than a billion light years away, spiralling towards one another and eventually merging. Since then, LIGO has detected several more black hole mergers, and in 2017 it allowed us to observe a pair of neutron stars smashing together for the first time. It was one of the most-observed events in scientific history. In just three years, gravitational wave signals have given us the first direct evidence that black holes exist, taught us how the heaviest elements in the universe are formed, and helped us measure the expansion of the universe.