LIGO detected the first directly observed gravitational waves in 2015.
The Laser Interferometer Gravitational-Wave Observatory, or LIGO, uses long vacuum tunnels and laser interference to measure tiny changes in distance caused by passing gravitational waves.
The first signal, called GW150914, came from the merger of two black holes. LIGO's two detectors in Washington and Louisiana recorded it on 14 September 2015, and the discovery was announced in February 2016.
Albert Einstein predicted gravitational waves in 1916 as a consequence of general relativity. The observation opened a new way to study violent cosmic events without relying only on light.