The dark blue Straits of Florida, where water depths average approximately 1000 feet (305 meters), contrast with the lighter blues of much shallower Biscayne Bay and Florida Bay at the southern tip of the mainland where depths average 6 to 10 feet (2 to 3 meters). The northern end of the Florida Keys extend north to southwest of the mainland. The cultivated fields between densely populated greater Miami and the darker, more uniform Everglades grow a variety of commercially produced vegetables and fruits. Discernible are the light, linear, single runway (slightly below middle) of Homestead Air Force Base east of Homestead and a large, roughly rectangular feature along western Biscayne Bay southeast of the air base that appears to be a fish hatchery or specialized aquaculture center. Discernment is difficult between elevated ground and swampy saw grass and between permanently water covered areas of sea grass and the swampy, vegetated coastal plains