A 3-silo complex. Can be accessed from the northern area of the Open Prairie. The silo doors can be busted down by attacking them. Inside, one of the missiles is still active and viable.
The complex populated by googlepedes, which are like centipedes but holy shit. There's also an underwater area with squids that will grab you, keeping you from moving until you kill them.
The underwater area is the key to accessing the majority of the complex. Pay attention to radiation signs!
A detailed map with descriptions can be found here.