It is...
trouble
Most are just pits or at best just chambers that go from one set of manholes to another. They quite often have to be pumped out to enter.
If they were actually communication tunnels then entering them is a federal offense. They're pretty strict on them as they still hold some 000 lines.
If caught in them, it's another level - not like being in a drain or abandoned building. It's way worse than been in the new metro tunnels and the like.
There's not much to see in the pits.
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