Laser Interferometer Gravitational-Wave Observatory. This is used to observe gravitational waves, and determine how they deform space-time. Each of the "legs" is a 2.5 mile long vacuum chamber. This is one of two LIGO detectors - the other is in Livingston, LA.
LIGO - the Laser Interferometer Gravitational-wave Observatory - is a new research facility nearing completion on the grounds of the Hanford Reservation, built to record otherwise undetectable gravitational waves striking the earth. The facility consists of a control building, from which a pair of two mile long, ground-level tubes project at a right angle from one another. A laser is projected through the near-perfect vacuum maintained within these long chambers, bouncing back to detectors in the control building. The facility acts like a giant optical microphone, tuned to gravity waves generated by distant celestial entities. It is a collaboration betweeen Caltech and MIT, funded by the National Science Foundation, and is coupled with an identical facility under construction in Louisiana.