black-water rafting
n. A sport that involves riding inner tubes on rivers that run through caves.

Example Citations:
The latest innovation from the folks Down Under — the people who brought you bungee jumping and jet boating — is called black-water rafting. It takes place underground, with wet suits and headlamps, and is as different from white-water rafting as eight-ball is from billiards. The adventure begins at New Zealand‘s Waitomo Caves, a two-hour drive south of Auckland on the road to Rotorua.
—Benjamin Epstein, “Black-water rafting: Not for the claustrophophobic,” Los Angeles Times, May 24, 1992

The Waitomo Caves are famous for their limestone caverns and glowworm grottoes. Most visitors do the conventional guided tour by boat — boring, frankly. The best way to experience these caves is by black-water rafting, not as wild as it sounds.

When I first heard of this adventure, I had visions of shooting rapids in the dark and ending up lost in some endless tributary miles underground. Actually, black-water rafting is a gentle experience, like traveling through the womb of the earth in an inner tube.
—Babs Suzanne Harrison, “Waitomo Caves: Tubing in the dark with glowworms,” The Dallas Morning News, May 23, 1993

Earliest Citation:
Speaking of rafts, have you heard of black water rafting? This is the latest in New Zealand, and it involves grabbing an inflated car tube and jumping down a water-filled cave.
—Michael Harvey, “Packages.” Herald, September 16, 1988

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