Irwin Allen's failed pilot promo for a show that was suppose to replace Star Trek, when that show got canceled. The pilot was re-shot and presented again in 1971, but this version also failed to become a TV series.
The pilot was made as a short promo that only highlighted what the eventual show would contain. The amphibians are, for instance, only mentioned, but not seen, and same goes for the underwater city itself, which can only be seen from the outside, as General Matthews submarine Titan approaches the city.
The short's producer and creator Irwin Allen had already made several sci-fi and adventure movies about the sea and that which lies beneath it, so, inspired by Star Trek's popularity, he intended to use this experience to produce a similarly-styled undersea sci-fi adventure series for television.