John Russell Fearn 
Operation Venus [EPUB ebook] 

Two pilots crash their plane in the Brazilian jungle, becoming the prisoner of an English explorer who is ruling by fear the superstitious unknown race who inhabit the city. The fabulous city, protected by scientific means, had given rise to the legend of El Dorado. These people had been created centuries earlier by Venusian scientists to tend the city and their many scientific machines whilst they placed themselves in suspended animation, embarking on a mental exploration of the cosmos. The Venusians had been obliged to relocate to Earth when their home planet was devastated by a plague. Then the Sleepers awake and the action switches to Venus, where they attempt to repopulate it.