what happen in chapter 18 and 20?

Chapter 18- After spending a night at the bottom of the crater, the men awake from the best sleep they've ever expereinced. From here, they will slowly continue to descend.

Chapter 20- The men are starting to get low on supplies. Water rations will only last another three days, and there was little hope of finding a spring to replenish their supply. Regardless, the men continued on their path, viewing history in the walls around them. At the end of the chapter, the Professor concedes they may have to turn back.