Fermilab's Tevatron was a landmark particle accelerator; at in circumference, it was the world's second largest energy particle accelerator (CERN's Large Hadron Collider is 27 km in circumference), until being shut down on September 30, 2011. In 1995, two teams at Fermilab (CDF and DØ, detectors which utilize the Tevatron) announced the discovery of the top quark.