Yuma County is home to two premier military installations, MCAS Yuma and U.S. Army Yuma Proving Ground. The military is the second largest contributor to the local economy after agriculture. MCAS Yuma is the busiest air station in the Marine Corps, and supports 80 percent of the Corps' air-to-ground aviation training because of its access to 2.8 million acres of training ranges and superb flying weather. Each year, approximately 100 aviation units deploy here for training, bringing about 14,000 personnel and 600 aircraft to Yuma for periods lasting from a few days to seven weeks. Yuma County's largest single civilian employer is the Yuma Proving Ground. At 1,300 square miles, YPG today is one of the world's largest military installations, hosting up to 100 tests simultaneously. Every year, tens of thousands of artillery, mortar and missile rounds are fired, 36,000 parachute drops take place, over 130,000 test miles are driven, and nearly 4000 air sorties are flown.