As envisioned by Burroughs, Opar is a lost colony of Atlantis located deep in the jungles of Africa, in which incredible riches have been stockpiled down through the ages.
The city's population exhibits extreme sexual dimorphism caused by a combination of excessive inbreeding, cross-breeding with apes, and selective culling of offspring.
Farmer conjectures on the inhabitants of Opar, and even goes to suggest that the city's populace was on the verge of extinction at the time of the events in the original Tarzan novels.
Tarzan himself returns to Opar, finds it empty or abandoned, tours the ruins, and (out of respect) decides to hide the city from modern curiosity by diverting a river to bury it in sediment.
In Farmer's vision, the interior of Africa was then occupied by two huge linked inland seas, which were the cradle of a pre-Egyptian civilization called Khokarsa.
Both were later completed by Christopher Paul Carey (known best for his work on the Pathfinder Roleplaying Game), and later the novels were all compiled into the omnibus Gods of Opar.
In the pre-title opening sequence for The Legend of Tarzan (2016), a fictionalized Léon Rom, portrayed by Christoph Waltz, led an expedition to secure the fabled diamonds of Opar to help finance the take-over of the African Congo for King Leopold II of Belgium.
Even after the release of the Leopard Men, La's spirit possesses Jane and she (briefly) revives the city and creates statue warriors until Tarzan and a native African shaman interfere.