Each detector looks like a big L, made up of two tunnels 2.5 miles long. It's designed so that if a gravitational wave passes by, it will stretch space along one direction of the tunnel and squish space along the direction of the other. The stretching and squishing changes the tunnels' lengths by a tiny amount, and that change can be detected by lasers.
…This is, arguably, the most direct observation of black holes ever made. Because black holes are (as their name implies) “black”, they can’t be seen with ordinary telescopes. Up until now, their existence has been inferred by looking at the stars and gas swirling around them. This gravitational signal comes directly from the holes, and it is virtually incontrovertible proof that the holes are out there. “If black holes didn’t really exist, you couldn’t explain these waves,” he says.