This is another of those curious activities to come out of New Zealand, often called extreme sports, though they are frequently not particularly extreme. It sounds like a relative of the older and better-known white-water rafting, in which small groups shoot rapids in fast-flowing rivers, though more usually in inflatable dinghies rather than on rafts. Black-water rafting is even more of a misnomer. It’s called that because it takes place on underground streams in the dark. Participants are dressed in wet suits and fitted with inflated inner tubes, which both buffer wearers against sudden upsets on slippery rocks and also keeps them afloat when they go down rapids and over waterfalls in the dark. A New Zealand tourism Web page says firmly that the correct generic name for the activity is cave tubing and that the other one, rather better known, is in fact the trading name of the firm that invented the whole crazy undertaking back in 1987.