A monofin is a type of swimfin typically used in underwater sports such as finswimming, free-diving and underwater orienteering. It consists of a single or linked surfaces attached to both of the diver's feet, emulating the fluke of Cetaceans like whales or porpoises. Even though the diver's appearance might be reminiscent of a mermaid or merman, monofin swimming is not the same as mermaiding.
A freediver using a monofin
Herbert Nitsch wearing a monofin at the Cyprus BIOS 2004 Freediving Open Classic
Monofin made in 1969 by Franco Pavone in Bologna
Monofin
Swimfins, swim fins, diving fins, or flippers are finlike accessories worn on the feet, legs or hands and made from rubber, plastic, carbon fiber or combinations of these materials, to aid movement through the water in water sports activities such as swimming, bodyboarding, bodysurfing, float-tube fishing, kneeboarding, riverboarding, scuba diving, snorkeling, spearfishing, underwater hockey, underwater rugby and various other types of underwater diving.
Full foot fins
By 1974, modern-looking swimfins in regular use in landlocked, "second-world" Hungary.
1959 Soviet postage stamp with image of finned recreational diver in tribute to the DOSAAF sport organisation.
Swim fin sole showing compliance with German standard DIN 7876:1980