Structures are dilapidated. Don't be fooled by appearances. Do not climb on, around, or under structures. Animals live in abandoned mines. Rattlesnakes, bats, bears, or mountain lions can den or escape the heat in the dark recesses of a mine.
Underground mines can be critical habitat for such species.
You should not disturb them. Bad Air is oxygen-deficient or toxic air that can build up in abandoned mines, causing dizziness or even unconsciousness. Bad air is odorless and tasteless. Carbon monoxide and carbon dioxide often collect in low areas or along the floor in horizontal workings. The motion of walking can cause bad air to mix with good air.