A long, long time ago, a pair of black holes collided with such power that they created ripples in spacetime, which emanated through the universe. All the while, molecules on a tiny rock in a fairly irrelevant nook of the Milky Way galaxy arranged themselves into living things, which evolved into self-aware apes. Those apes eventually realized they could actually measure those spacetime wobbles, and built several kilometer-long machines (tiny if you really think about it) in order to do so. When they flipped the switch, they caught the wobbles just in time.
Source: Physicists Just Spotted Gravitational Waves Again—So What’s Next?