The Caves of Drach is a juvenile science fiction novel, the seventeenth in Hugh Walters' Chris Godfrey of U.N.E.X.A. series. It was published in the UK by Faber in 1977.
Plot summary
A multi-billionaire's grandson gets lost in the Caves of Drach in Majorca. The grandfather employs the team of astronauts (who are on holiday) to investigate. The expedition finds him in a huge and airy underground civilization peopled by practically immortal age-old humanoid beings who hail from the stars and took refuge underground when a Terran Ice-age threatened. The 'Cenobians' have a utopian civilization and avoid contact with the surface to keep it that way, and yet their advanced technology could bring an end to famine and disease.
References
The Caves of Drach Wikipedia(Text) CC BY-SA