A pure pulp era classic! Hunted by German U-Boats in the Atlantic, experimental submarine X-111 disappears. Five years later, an unusual man visits the Navy Department, telling a strange story: that at the bottom of the ocean exists a city of wonders, inhabited by people of great intelligence -- and haunted by a terrible past that threatens them again. Originally a 1927 serial, and reprinted from a 1948 edition illustrated by Charles E. McCurdy.