Tarzan and the Forbidden City

Tarzan and the Forbidden City was published in 1938 and revives the lost-city formula one more time. The forbidden city of the title is Ashair, located inside a vast central African lake, accessed through underwater tunnels that only the Ashairians know. Various expeditions have been searching for Ashair for years, looking for the legendary Father of Diamonds, a single enormous gem reputed to give immortality to its possessor.

Tarzan becomes entangled with several competing expeditions, all of which want the diamond for different reasons. An American adventurer named Brian Gregory has gone missing inside the lake city. His sister Helen Gregory hires a separate expedition. The plot becomes a layered confrontation between rival treasure-hunting groups, all trapped inside or below the lake. The Father of Diamonds turns out to have surprising properties that Burroughs takes seriously rather than treating as a McGuffin. A late-period lost-city entry with stronger plotting than most.