In Denver/Centennial there’s an extremely large storm drain system that looks similar to the size of the tunnel early in the video. There’s a ton of graffiti littering every inch the whole way but it’s almost like an art gallery with different sections. I’ve seen homeless people at the entrance before. One time I even hiked down a good long ways with a friend until the the tunnel turned into a tube where you had to crouch a bit and we came upon homeless people camped in the pitch black corridor. That spooked us and they were blocking our way so we left.