John Cleves Symmes Jr. (1780-1829) was an American army officer and geologist known for his unconventional theory about the Earth. He proposed that the Earth was hollow and contained inner concentric spheres, which he believed could be inhabited. His ideas gained some attention in the early 19th century, leading to public lectures and writings. Symmes' theory inspired various explorations and even fictional works, including those by authors like Edgar Allan Poe and Jules Verne. Although his hollow Earth theory was ultimately discredited, it remains a fascinating part of American scientific history and popular culture.