Nowadays, it's so common to call the first episode of a series the "Pilot" that most people don't even realize that's not what it is. A "Pilot Episode" is basically a proof-of-concept for a series, to show network executives and studio types in order to get them to pick up your show. There's no set length or format for these things. Sometimes they take the shape of a TV movie (As in Babylon 5: The Gathering or Legend of the Rangers), sometimes they're episodes of a show that's already running, as an attempt to create/sell a spinoff (As in the TOS episode "Assignment: Earth" or the Battlestar Galactica episode "Message from Earth." - these are called "Backdoor Pilots," which sounds dirtier than it actually is.) Sometimes they're just a short film to give a rough idea of what a finished show would be like, if they don't have the budget to do it. These don't have to be episode length, as they're never intended to be seen by the public - the "Gilligan's Island" pilot was only 15 minutes long, for instance. In nearly all cases, Pilot Episodes are rough drafts of the series to follow, and there's generally a lot of changes in cast, look, and sensibilities between it and the series to follow.
Basically, the really short pilots are there to test the waters are there just to judge the interest of the networks. If they like what they see, then they go on with a longer test in the future. If they don't like what they see, then the producers get to walk away without having blown a huge investment of time and money.
This, then, is one of these "Short Pilots" for a TV series Irwin Allen was trying to sell in 1970 called "City Beneath The Sea."
and if that doesn't work, go here http://www.youtube.com/watch?v=dZrifEjU-H0
It's pretty weak. The network was right to pass, and I doubt the Sid & Marty Croft-level special effects really won anyone any friends. I kept expecting a slestack to come loping through. It does, however, have a young James Brolin and genre-fave Lloyd Bochner (Who, come to think of it, was in "Message From Earth.") and the Chinese Security Cheif is super-hot.