Hollow World wasn’t strange from either perspective. There have always been fantasies involving the hidden civilizations under the earth. There was also among designers a fetish for the pulpy adventure stories. See also, Doc Savage, the Nile Empire for TORG, Justice Inc and Lands of Mystery, various Buck Roger’s iterations, Thrilling Places, Hollow Earth Expeditions, Spirit of the Century, as well as a few dozen others. Hell, even Traveller is based on the pulpy SF like Dumarest and Lensman.