Google - Hawthorne Army Depot. All those bumps are ammunition bunkers - earth covered concrete arches called igloos. Everything you ever wanted to know; construction, history, current usage.
The Navy built a number of large inland depots just before and during WWII. The Navy had an unpleasant experience at Lake Denmark (Navy facility at Picatinney Arsenal, NJ in the mid 20s so wanted to spread out the risk to any one installation. Lake Denmark was leveled- trees, buildings, you name it. Hawthorne’s counterpart in the east is Crane Army Ammunition Activity in Indiana. Also see for the central US; McAlester Army Ammunition Plant, McAlester, OK and a closed facility in Hastings Nebraska. Another large rocket motor facility (and other items) was built in Camden, Arkansas. The Camden facility is home to dozens of munitions contractors. The site already has most explosive safety features built into the design.
These depots/manufacturing plants/testing sites were taken over by the Army in the later 1970s when conventional ammunition was moved under the Army. Specific munitions like missiles, underwater mine, and torpedoes were retained by the individual services.
The currently open ones; McAlester, Hawthorne, and Crane; still have Navy components operating on the installations (Surface Weapons Centers and the like).