We had descended five thousand six hundred feet: more than an English mile. The center of the Earth is still four thousand miles below us. The tunnel now changes direction, so we can move more quickly.

If we are going all the way to the center, then this shallow angle means we may have to walk many thousands of miles. Should we walk as fast as possible, over these unfamiliar rocks? Or be slow and careful, even though our food will run out after two months? Or should we try something else?