"The LIGO “observatory” is made up of two identical and widely separated interferometers situated in sparsely populated, relatively out-of-the-way places: LIGO Hanford in southeastern Washington State in an arid shrub-steppe region crisscrossed by hundreds of layers of ancient lava flows; and LIGO Livingston, 3002 km away in a vast, humid, loblolly pine forest west of Baton Rouge, Louisiana."
Soon to be built in Italy, Germany, Japan, and India.
LIGO was designed with two detectors so far apart for good reason. LIGO’s detectors are so sensitive that they can 'feel' the tiniest vibrations on the Earth from sources very nearby to sources hundreds or even thousands of miles away. Things like earthquakes, acoustic noise (e.g., trucks driving on nearby roads, farmers plowing fields, things that people can hear and feel) and even internal laser fluctuations can cause disturbances in the detector's laser that can mask or mimic a gravitational wave signal in each interferometer.
If the instruments were located close together, they would detect the same vibrations at the same times--both from Earth-sources and from gravitaitonal waves and it would be nearly impossible to distinguish local noise from a vibration from deep space.
Facilities far apart, however, will not feel the same local vibrations but they will feel a gravitational wave vibration at the same time. By comparing data from both sites, scientists can ignore the vibrations that differ between the sites and look only for identical signals that occurred at the same time at both locations. This is why two or more detectors are essential.
Each one acts as a noise filter for the other, leaving only signals from gravitational waves as the stand-outs. Without working together in this way to confirm each other's detections, gravitational waves could never be positively detected with an interferometer like LIGO.
LIGO’s collaboration with VIRGO (I think that this will be new one in India) adds a third interferometer to the mix thus significantly increasing the confidence that a detected signal is real.
