First, listen for the air movement. It tells you where the structure is weaker or where there might be hidden shafts. Next, check for water marks—those spots often mean the concrete has softened and you can find easier passages. Keep an eye on any graffiti or patches; people tend to cover cracks, so that can reveal where old holes were. And finally, track the old ventilation shafts; they usually lead to more open spaces or to where the tunnel’s load is lighter. That’s the most reliable way to spot good spots.

Yeah, rats and pipes are the usual. Stick to the vents that are still functional; they’re the ones the system keeps running. If the pipes are broken, that’s a sign the load’s shifted, so a bit of caution is still worth it. Good luck, and keep the ears open.