When I was driving bags from concourse C to concourse A for a connecting flight (international) I used the underground tunnels the whole way. This was a regular job for me that I enjoyed doing. It was time alone and it let me drive the CNG tractors with only one trailer (backing up with multiple trailers is a bitch).

Going through the tunnels at night, I saw a lot of crazy stuff. Weirdest was definitely what looked like a bonfire with people around it in one of the alcoves. On many occasions, I saw lone people walking in the halls too small for the tractors to go through.

I asked one of the supervisors about it and he said I was seeing stuff. It was very common since the airport was built over an Indian burial ground, which is true.

Little known fact about that airport: there are 3 concourses in use. There are also other tunnel systems for more concourses already built. One of my biggest regrets was not taking a tractor into Concourse D and above when I was on my break or something. Using company vehicles to go exploring in unused parts of the airport was something I only could have done when I put my 2 weeks in already.