Gary Paulsen, Woods Runner

From the text:

The forest was unimaginably vast, impenetrable, mysterious and dark. His father had told him that a man could walk west for a month, walk as fast as he could, and never see the sun, so high and dense was the canopy of leaves.

Samuel had heard stories of that happening to some men. They entered the forest to hunt or trap or look for new land to settle and simply vanished.

Some, he knew, were dead. Killed by accident, or panthers or bear or Indians. He had seen such bodies. One, a man mauled to death by a bear that had attacked his horse while the man was plowing; the man’s head was eaten; another, killed by an arrow through the throat. An arrow, Samuel knew, that came out of the woods from a bow that was never seen, shot by a man who was never known.