He joined the United States Army in 1802 and rose to the rank of Captain during the War of 1812. In that war he served on the Canadian frontier, at both Fort Niagara and Fort Eire.
His 1818 theory of a Hollow Earth, envisaging a shell about 800 miles thick, with openings at both poles about 1400 miles across, made his name famous/notorious. He proposed the name Symzonia for the inner territories.
See a current reprint of his book: Symzonia; Voyage of Discovery, ISBN 0405063121.
Compare a fictional echo of Symmes in Ian Wedde's Symmes Hole (1987).