After their expedition to Saturn, Chris Godfrey and his friends were given the longest spell of leave they had ever had. Every day they expected to hear about their next assignment from Sir George Benson, Director of the United Nations Exploration Agency, but when they tried to get in touch with him they found it was impossible. Clearly something strange was going on.

When Sir George finally reappeared he had a startling proposition for them. A new kind of expedition was to be launched, not into space but into the depths of the earth. The astronauts were about to become 'subterranuts'. Or rather one of them was, for only one man could enter the capsule which was to carry him down the Mohole, the borehole which had been drilled twenty-one miles into the earth, to end in a huge underground cavern...