Thirst!
The Professor will not accept they have chosen the wrong route and walks on.
That evening, Harry goes to bed thirsty. He wonders if this is really the right way and if they will find a fresh water supply so deep beneath the surface of the earth.
The next day, the last drop of water is drunk and Harry begins to suffer horribly. He eventually collapses to the ground in a dead faint. Somewhere nearby he hears the Professor whisper "All is over," and as the light dims, he sees the Icelander making his departure, lamp in hand.