Its like Area 51 EXCEPT the public can tour it, there is a public airport, sections are owned by the local electricity company and there isn't much that is secret or mysterious about it. There is even a Chinese owned wind farm on the property. Most of the Military use of the complex was discontinued in the 60's. There is even a several thousand acre Wildlife refuge. and an 18th century cemetery.
Yes, there are areas not open to the public, most of that is due to the health risks from unexploded munitions and pollution problems left over from the decommissioned Boardman Coal plant that used to provide Electric power to the region.