(CNN) — Shayne Burke let out a scream. A grizzly bear had just taken a “good chomp right to the bone” on his leg. The bear dropped him. “And then she was up on my back, and then just went to bite down ...

There's nothing worse than staring down an angry grizzly bear. Okay, I take it back. The one thing worse than getting caught by an angry grizzly is getting caught with a cliff against your back.

Imagine drifting along a wide, powerful river on a calm rafting trip, completely surrounded by untouched wilderness. The mountains rise around you, the trees sway in the breeze, and wildlife moves ...