Welcome to a piece of Cold War history! The large building nearby was constructed to house the Automatic Voice Network, or AUTOVON, and is owned by the Inter-Community Telephone Company. The network was designed in 1963 as a military phone system to be used in case of nuclear war.
Here is an excerpt about AUTOVON and the history of the building from the ICTC website:
Sometime in 1964, John Nilsen (then owner of ICTC) learned from a friend on the West Coast that a military switching center was to be located near Wheatland. Nilsen then contacted the Defense Department about the possibility of ICTC servicing the switching site, as it would be located within the company's franchised area. At the same time, Northwestern Bell was making plans to service the site and obtained an option to purchase ten acres of land just outside Wheatland. Northwestern Bell would be providing toll service for the switch no matter who serviced the site.
The decision as to who would service the site was long and painful. The North Dakota Public Service Commission determined that either company could service the site since it would be in ICTC's service area and Northwestern Bell would be providing the toll service. The final decision would have to be made by the contractor, the United States Government. During the next two years, Nilsen made several trips to Washington, D.C. to appear before Congressional committees and finally, to testify at a Justice Department hearing. In pleading his case, Nilsen had to convince his listeners that ICTC, although small, was just as capable of providing service to the site as a larger company.
ICTC was awarded the contract, and on April 4, 1971, the CONUS AUTOVON (Continental United States Automatic Voice Network) Switching Center was put into service as a part of the U.S. Department of Defense worldwide defense communications network. The Wheatland Autovon remained operational until February 28, 1996, just one month short of 25 years of service.
The building was built to house the switches and telephone gear. To do so, it had to survive a nuclear war and the resulting radioactive fallout. The walls are made of 18-inch thick reinforced concrete and the roof is made of pre-stressed concrete with another 10 inches of poured concrete on top of that. There were originally only two doors, both lead-lined, with the front door leading into an airlock arrangement that goes into a decontamination shower. The other door is a large service door on the north side of the building. The small porthole and large garage door on the west side of the building were cut in later by ICTC. There are two large diesel generators (think semi truck engines), with one large diesel fuel tank inside and two bigger ones buried outside. There is also a large battery bank for more emergency power. The air handling system has filters the size of an adult and is strong enough to pressurize the entire building, creating a positive pressure to keep out contaminated outside air (higher pressure inside than outside). The building has its own water well with a reverse-osmosis filtering system. The generators, batteries, air filters, and well make the building entirely self-supporting. Finally, I have it on good authority that the original military K-rations are still inside, and possibly edible (but I don't think I will try one).
The building now serves as the local ICTC headquarters with all their internet and phone switching gear, as well as their trucks and repair equipment. The small building on the SW corner of the property was originally built by AT&T (Northwestern Bell from above) to service an antenna that no longer exists. ICTC currently uses it for storage.
Please park on the side of the gravel road, and don't block the driveway in case ICTC needs to get trucks in and out. The cache is a small ammo can in an interesting place. You DO NOT have to enter any buildings.
Wiki page on the AUTOVON system.
Another neat page about AUTOVON.