True story: I was once almost kicked out of BYU for attempting to sneak into one of the tunnels under the campus.

A couple of years later, a professor actually invited me through the door I had tried to enter. It turned out that what was inside was a gymnasium-sized physics lab that was directly under one of the larger quads on campus. The major feature of which was a particle accelerator. That fact might have been known among the physics majors, I don't know, but it certainly was not common knowledge among the student body at large.

There was another "secret" door on campus, this one into the hill that the Honors Department main building sat on. That door led to the nuclear reactor. Nobody ever tried to get in there.

For a private, mostly undergraduate university, it had a surprisingly active nuclear physics research group.