The day spacetime rang

Two black holes had been merging for a billion years, and at 09:50:45 UT the wave arrived and stretched every instrument in Louisiana and Washington by less than the width of a proton.

It was the first direct detection of a gravitational wave, and it turned up during an engineering run, before the observatory had formally started looking. The same trick — bouncing light off distant mirrors and timing it to absurd precision — is how we measure the Moon's retreat, using reflectors Apollo left behind.