An ancient, sunken continent—located somewhere in the Pacific Ocean—whose sole surviving "remnant" was "a glass-enclosed city of ancient, weird design" which could be raised to the surface and then submerged like a submarine and which served for a time as Lex Luthor's base of operations.
"You'll admit it was a miraculous achievement!" boasted Luthor. "Working underwater, I raised a glassolite-dome over the city, drained out the water, then raised the city to the surface of the ocean."
Pacifo was destroyed by Superman in Spring 1940 (S No. 4/2)
(See also Atlantis)