The Forest of Lost Men is one of the last forests on Barsoom. It is located at the equater.
The forests houses two rivalling cities: Invak and Onvak. The inhabitants of both cities have developped special pills that render them invisible for about a day, except when exposed to the light of a special lamp. They are constantly at war with each other.
The forest is seen in the final story of Llana of Gathol, when John Carter and Llana end up there on their way from the North Pole to Helium, and are taken captive.