Sept. 14, 2015, was one of the most important days in science history. It marked the first-ever detection of gravitational waves, tiny ripples in space-time (the four-dimensional union of space and time), a milestone notched by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Since that day, LIGO — composed of two highly sensitive laser interferometers located in Hanford, Washington and Livingston, Louisiana — has been joined by two smaller gravitational wave observatories: Virgo, which came online in Italy on Aug. 1, 2017, and the Kamioka Gravitational Wave Detector (KAGRA) located in Japan, in late 2019.

Over the course of four operating runs, separated by shutdowns to allow for improvements and upgrades, the LIGO-Virgo-KAGRA instruments have become so sensitive that the

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