Tony, Chris, Money and Serge, having explored the limits of our immediate Solar System in previous books, now face a quite different challenge only twenty miles away from topsoil. The mohole is an exploratory shaft, the deepest yet attempted by scientists, designed to probe the boundary between earth crust and mantle, the Mohorovicic Discontinuity. The astronauts' mentor, Sir George Benson, has been working on the project and asks the boys if one would be willing to try his luck as a subterranaut. All would but Serge is chosen and the Russian is dropped twenty miles into a huge cavern where he runs into an unforeseen dilemma. And his fragile craft is permanently undergrounded. While Chris and Tony (Morrey is too fat for the two fÃ¡ot hole) frantically prepare for a rescue, Serge is faced with microcosmic fungus, fatal bacteria, and huge, menacing, mobile eggs. Even a young spelunker may find a believability gap here but there is, again, a neatly intergrated bulk of material on earth science and biology to keep them plunging ahead. Even through such fearsome cliches as ""intrepid explorers"" and ""fearsome depths.