urban caving
n. The unauthorized exploration of tunnels, drains, and other features found beneath a city.
Other Forms
He said that not only were there immediate dangers but Sydney sewers were also built to overflow into stormwater drains, which presented a raft of health risks.

"The closest we'd get to urban caving is a couple of sea caves — certainly no stormwater drains," he said.
—Kara Lawrence, “Tunnel vision,” The Daily Telegraph, January 22, 2008
A mask seized by police was not, as police alleged, a device to "disguise face with intent to commit an indictable offence", Mr Gibson said.

Brandishing photos of his client wearing it, he explained that Sanders was into "urban caving", that he and his mates went exploring under the city in sewers and tunnels.
—Neil McMahon, “Light at the end of the tunnel for a man up to his neck in it,” The Age, December 21, 2005
1996 (earliest)
Anyone else out there into urban caving? I realise this newsgroup isn't really about this kind of activity, but it looks like it's being taken over by spam anyway. …

University steam tunnels, storm drains etc etc. Please post your experiences. Also, if you live in Vancouver, please post locations of any known tunnels.
—“Urban Caving,” alt.caving, March 15, 1996
Urban caving is part of a larger set of activities known as urban exploration (often shortened to urbex or UE), where the goal is to access and explore those parts of the city that are normally off-limits to the public. Since this almost always requires unauthorized access to these areas, urban exploration is also called infiltration or creeping. Other synonyms are building hacking and roof and tunnel hacking. Urban caving is also called draining, sewering, or urban spelunking. Note, too, that the earliest citation is actually a quote from an earlier newsgroup post that seems to have been relegated to the bit bucket.