A Safeguard ABM system missile site radar. (Public domain photo by Craftsman2001).
Here, you go, RAW fans -- a big "eye in the pyramid" you can look at from behind a fence, conveniently located in Nekoma, North Dakota. It's part of an ABM system built at great expense and quickly abandoned. Read about it from Atlas Obscura.
Meanwhile, a local jobs agency has bought the site, which includes less interesting stuff besides the pyramid. "The Job Development Authority is hoping a big name company like Google will move servers into the pyramid." Meanwhile, Wikipedia has a history of the program, if you are curious.
Hat tip: @Stargazer_KEA on Twitter.