I'm always finding a John Wyndham novel I haven't read before. And yes, I did find this in a used book store. This was John Wyndham's first novel, originally serialized in a magazine, written when he went by the name John Beynon.
Published in 1935 but set in 1964, "The Secret People" takes us to a place intruders never leave. After Mark Sunnet's rocket plane crashes in the Sahara Desert, which is being turned into a "New Sea" by France and Italy in a monumental feat of engineering, he and his girlfriend Margaret find themselves prisoners of a people determined to keep their existence secret. Hence the title of this book.
These short-statured people (who resemble white pygmies) dwell in an underground network of vast caves and are, on the face of it, mired in primitivism. The caves are lit by luminous globes of unknown power, suggesting that this civilization was once highly developed technologically but is now long past its time of glory. While Margaret and her cat become a focus of worship, Mark is thrown in with the other prisoners. These are people of various nationalities who were unfortunate enough to stray into the pygmie's domain over the years - destined to live out their lives subsisting on the fungus of giant mushrooms which grow in the caves. While many are slumped in apathy, some of the captives have preserved their sanity by working on an escape tunnel. The rising water levels have heightened the sense of urgency.
Because Wyndham was a fan of H.G. Wells, it is reasonable to guess he had read "In the Abyss", a story with a similar theme. The characters are an ever-resourceful set, which, apart from Mark and Margaret, include a couple of can-do Americans to boost morale, an English academic speculating on the history of the pygmies, a slimy villain, a good-natured African giant, and, to represent the pygmies, a philosophical priest. Wyndham's style in the 1930s is different to his more acclaimed style of the 1950s, but it remains readable today, even if it is a little dated.