Virgo is a gravitational-wave detector in Italy, the primary detector of the European Gravitational Observatory. It is a large laser Michelson interferometer sending two beams back and forth through 3 km vacuum chambers, reflected multiple times to travel 100 km each, designed to detect minute changes in distances due to gravitational waves. It aims at the 10-10000 hertz frequency range, to detect mergers of compact objects.
The detector began operation in 2007 with no detections during its first years. Following an upgrade completed in 2017 (aka Advanced Virgo), it was in operation at the same time as LIGO, resulting (as intended) in two 2017 detections seen by Virgo and both LIGO detectors: GW170814 and GW170817. In the latter, Virgo detected "something" but the detection wasn't clear. Given that these detectors' sensitivity depends upon the direction of the source of the signal, this fact helped localize the source and contributed to its discovery in visible light. Following another upgrade completed in 2019, more than 20 additional candidate GW detections have been reported by both Virgo and LIGO.