The Eltanin Antenna is an object that was discovered and photographed by the USNS Eltanin, an Antarctic oceanographic research ship, in 1964. The object, which is a regular antenna-like structure found on the sea floor, has sparked controversy and speculation among proponents of fringe and UFO-related theories. Some believe that the object may be an extraterrestrial artifact. However, other authorities have suggested that it is actually an unusual carnivorous sponge known as Chondrocladia concrescens. The identification of the object as a sponge was largely unknown until recent years, when it gained attention on a UFO mailing list.
https://en.wikipedia.org/wiki/Eltanin_Antenna