On 14 September 2015, they (LIGO physicists) picked up a relatively big change in their Livingston lab in Louisiana, what you'd call a blip in the system. Then, 7 milliseconds later, they detected the same blip with their lab in Hanford, Washington, 4,000 km away, suggesting that it had been caused by a gravitational wave passing through Earth.
In the months since, researchers have been rigorously studying this signal to see if it could have been caused by anything else. But the overwhelming conclusion is that the blip was caused by gravitational waves -- the discovery has statistical significant of 5.1 sigma, which means there's only a 1 in 6 million chance that the result is a fluke.
In fact, the signal almost perfectly matches up with what scientists predicted gravitational waves would look like, based on Einstein's theory. ...
Ah, waves. Got it.