Mysterious bunker - there are 100's of them
The mighty grain elevator
Yes, the bunkers were part of a WWII Naval Ammunition Depot. Here is a note from Wikipedia:
"The former Naval Ammunition Depot (NAD) (near Hastings, NE), is one of Nebraska's former four major ammunition plants: the Cornhusker Ordnance Plant, the Nebraska Ordnance Plant and the Martin Bomber Plant. Its construction began in July 1942 on 49,000 acres (200 km2) and was completed in early 1943 with over 2000 buildings, bunkers, and various other types of structures. The cost of construction was over $71 million. The navy built in this location due the proximity to the area's three railroads, the abundance of underground water, cheap natural gas and electricity, the stable work force and the distance from either coast (being beyond the range of Japanese or German bombers). At one point during World War II the facility was producing over 40% of the U.S. Navy's munitions. It manufactured 40mm shells, 16 inch projectiles, rockets, bombs, depth charges, mines and torpedoes. Production peaked in June–July 1945, when the depot employed 125 officers, 1,800 enlisted men, and 6,692 civilians."