The Sahara is being flooded to create a new sea when the protagonist of the novel, Mark Sunnet, crashes his private rocket plane into an island of what is currently little more than a large lake. He soon finds himself, his female companion, Margaret Lawn, and a stray cat they call Bast, sucked into a cavern where they are promptly captured by mysterious pygmies. The diet of little people is centred on large fungi; the captives speculate that stories which reached the surface of the little people and their giant mushrooms may have led to the myth of gnomes. Sunnet finds that a tiered community has evolved in the caverns — the pygmies inhabiting a large underground collection of natural and artificial caverns and tunnels, and the captured humans in a deliberately isolated subsection of the caverns. He is also surprised to learn that family life exists in the caverns — "natives", children of captured humans who were born and have lived all their lives in the caverns exist, and are generally happy with their life, and have no wish to escape. By virtue of being accompanied by Bast, the pygmies consider Margaret to be divine and keep her isolated in a separate area of the caverns. Most of the captured humans do wish to escape, and two different methods are being tried. Both are tunnels, one going up at an angle, to try and break through to the surface, and another on a level, hoping to intersect a pygmy tunnel or cavern, from where they will be able to make their way to the surface. The pygmies are distressed, and it is Sunnet's arrival that reveals why to the captives — the pygmies fear their environment will be flooded and destroyed by the newly formed Saharan Sea. Their fear is well founded, and the waters break through into the pygmy caverns, eventually flooding the entire ecosystem. Sunnet, Margaret and Bast, and a handful of others survive. The story finishes with sunburn after years of subterranean life, and the establishing of a new company based on primitive — but unique — technology the escapees brought with them from the caverns.
Predictions
Set in 1964, the novel correctly identifies Queen Elizabeth II as the reigning monarch of Britain, although she was only third in the line of succession when the novel was published in 1935 and wouldn't be queen until 1952. An early passage in the book describes the comic-military reactions of Germany towards potential violations of their airspace by the protagonist's descending rocket plane. This suggests Wyndham considered it possible that the Third Reich would survive at least another 30 years into the future; an expectation of totalitarian regime longevity that is mirrored by his similar projections of the continued existence of the Soviet Union into the 21st century in The Outward Urge. The novel also references the Piltdown Man, which had not yet been exposed as a hoax at the time of publication.
