I'm hanging from a rope, high above the churning froth of an ice-blue river. My friends are waving and shouting out to me, but the roar of the waterfall muffles their voices. I pull myself off a wooden seat and lower my legs. Now there's nothing between me and the water below but crisp mountain air. Then I remember my guide's advice: "When you let go, the key is to look straight up," he says with a wink. "That way, you'll fall straight down." I squint at the sky, let go of the grip and plunge into the foaming pool below.

By the end of the afternoon I will have dodged boulders, careened through natural stone chutes and dropped from one of Gunma Prefecture's many massive waterfalls.

It's my first try at canyoning, a hybrid sport that combines swimming and rock climbing with the thrills of rappelling and rafting. One of the world's fastest-growing outdoor activities, canyoning claims enthusiasts from Switzerland to South Africa. Clad in wet suits, life jackets, helmets and climbing harnesses, practitioners work their way downstream, negotiating rocks, whirlpools and waterfalls. Think of it as being like white-water rafting, but without a raft.