Cave Clan does that kind of stuff in Australia. Although quite illegal, they're recognised as being useful for mapping long-disused and barely remembered tunnels under the cities.
Reminds me of back in the day at the University of Michigan, exploring the steam tunnels under the campus. We had a copy of the map, which got passed around like old lore from class to class.
During my time, they installed motion detectors and started arresting students, in part due to the media hype of James Dallas Egbert III, who the media blamed his excursions into the tunnels and eventual suicide on his involvement with Dungeons and Dragons role playing.
The kind of thing we loved doing as kids, but for which I would kick my kids' ass if I knew they were doing it.
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