In the wilderness, one false step can make the difference between a delightful respite and a brush with death.

On a beautiful summer afternoon in 1998, Dan Stephens, a 22-year-old canoeist, was leading a trip deep into Ontario's Quetico Provincial Park. He stepped into a gap among cedar trees to look for the next portage--and did not return. More than four hours later, Dan awakened with a lump on his head from a fall and stumbled deeper into the woods, confused.

Three years later, Jason Rasmussen, a third-year medical student who loved the forest's solitude, walked alone...