GEO600 is a ground-based gravitational-wave detector near Hannover, Germany. It is the product of the GEO Project, a German/British collaboration to detect gravitational waves. It was constructed in the 1990s, first taking data around 2002 and has since undergone improvements. Its basic design is similar to LIGO, a large Michelson interferometer, in GEO's case with 600 meter arms. (LIGO's are 4 km and Virgo's are 3 km, the additional length contributing to their greater sensitivity.) GEO600 has no unequivocal GW detections, but has value in developing and testing technology and gaining knowledge, which has contributed to LIGO and Virgo.