LIGO is the Laser Interferometric Gravitational Wave Observatory and consists of two classic interferometers on either side of the continental United States. The interfermoeters started initial operations in 2001. The interferometers work by detecting minute movements in the arms of the interferometer – two interferometers are needed to confirm the detection of a gravitational wave, and rule out local effects such as small earthquakes or instrument variations.
More information can be found at the LIGO home page.

An aerial photograph of one of the LIGO interferometers.
Credit: LIGO Laboratory