Who or what is the greatest danger to the main character?
The greatest danger to the anonymous man in "To Build a Fire" is not precisely the terrible cold but the danger of getting wet. Jack London wrote from personal experience. His story is a good illustration of the rule that a writer should write about what he knows. The reader learns a lot about the perils of the bitterly cold weather and the bleak, lonely setting. The average reader would not think that there could be any liquid water at that temperature of seventy-five degrees below zero. But London explains how it was possible.
The creek he knew was frozen clear to the bottom—no creek could contain water in that arctic winter—but he knew also that there were springs that bubbled out from the hillsides and ran along under the snow and on top the ice of the creek. He knew that the coldest snaps never froze these springs, and he knew likewise their danger. They were traps. They hid pools of water under the snow that might be three inches deep, or three feet. Sometimes a skin of ice half an inch thick covered them, and in turn was covered by the snow. Sometimes there were alternate layers of water and ice-skin, so that when one broke through he kept on breaking through, for a while, sometimes wetting himself to the waist.
When the temperature was seventy-five degrees below zero, as it is in this story, the water would freeze instantly if a man broke through the snow into a pool of water. He would be in extreme danger of frostbite and would have to build a fire immediately to dry out his wet pants, shoes, and stockings. London first explains the danger of breaking through the snow into one of the many "traps," and then narrates of how the man, in spite of his precautions, eventually does break through.
And then it happened. At a place where there were no signs, where the soft, unbroken snow seemed to advertise solidity beneath, the man broke through. It was not deep. He wetted himself half-way to the knees before he floundered out to the firm crust.
This is only the beginning of the man's troubles. His cold hands make everything increasingly difficult for him. He builds a fire to dry out his foot gear. But he makes the mistake of building his fire under a spruce tree that is so heavily laden with snow that it could barely hold another flake. His fire and his activities gathering twigs under the tree cause the tree-branches just enough agitation of precipitate an avalanche. The snow at the top falls onto the branches just below, and then the load of snow from both sets of branches fall onto the branches immediately below them. There is a veritable avalanche by the time all the snow gets down to the bottom. The man is covered with snow and his precious fire is totally obliterated. Then when he tries to build another fire away from the trees, his fingers are so numb that he can't even light a match. Eventually he freezes to death out there in the white wilderness.