These can be metal or concrete pipes that are typically either used to route streams underground, or to serve as drainage pipes in commercial areas. In this particular case, I attended a geocaching event (coffee and donuts) this morning at a local establishment to celebrate someone's 3,000th find. At the event, people were talking about going afterwards to find the cache that was hidden "under the parking lot". I had to take part in that, even though I was not dressed for it.
Making our way down into the large drainage area where the pipe entrance was, it wasn't hard to find what we needed to do.
Being eager to do one of these, I plunged in first, crawling on my knees while straddling the trickle of ice cold water in the center. All was well until I got about forty feet into the pipe, where it opened into a tiny concrete room. At this point I stopped and asked if anybody had a flashlight, which somebody trailing the pack did (you'd be amazed at what geocachers carry around...). I didn't mind the closed-in space or the dark, but not being able to see if there were snakes about was an issue for me (despite it being the middle of winter and 25 degrees out). The little room proved to be a relatively clean and dry 6 foot cube, which 3 of us crammed into, with 3 more in the pipe. We made the find easily, and then had a good chuckle when we realized that 6 experienced geocachers didn't have a single writing implement between the group. Somebody fetched one, we signed, and we were on our way.