Laser Interferometer Gravitational-Wave Observatory : A detector project for the measurement of gravitational waves in the United States, which was upgraded to the Advanced LIGO between 2010 and 2015 .The detector includes two interferometric gravitational wave detectors, with an arm-length of four kilometers each. One of them is located in Hanford, Washington, the other one is located in Livingston, Louisiana. The first ever direct measurement of gravitational waves was made at LIGO.

> LIGO website