I would have suspected that Sealab 2020...
...was based on the movie The Neptune Factor (1973)
...were it not for the fact the this animated series preceded it.
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"Sealab 2020" was the next to last of the Hanna-Barbera produced action adventure shows that was part of NBC-TV's Saturday Morning schedule from its premiere on September 9, 1972 until December 2, 1972. The series produced 13 episodes.
Only the original episodes aired from September of 1972 until December of 1972.
From January of 1973 until September of 1973,it was seen in re-runs.
Irwin Allen in fact had a film City Beneath the Sea (1971),which tread some of the same territory just a few years before.share
City Beneath The Sea was originally a made for TV movie that aired on ABC-TV in mid-1971.
It was produced by Irwin Allen of course......
With some stock footage that was used from his TV-series Voyage To The Bottom of the Sea.
It also had a reference to the 1970 science fiction/underwater adventure film
"Captain Nemo and the Underwater City" that starred Chuck Connors and Robert Ryan
and was produced by Ivan Tors.
Originally,the short-lived animated series "Sealab 2020" was suppose to be produced for CBS in 1971,but the network scrapped it off its Saturday Morning schedule in favor of "Pebbles and Bamm-Bamm" for the 1971-1972 season.
There were also talks under Hanna-Barbera for a live-action/adventure series to be produced for prime-time for CBS(the first for the company,and the first series under Hanna-Barbera to be produced for prime-time). Those plans were eliminated.
Reason? Too expensive to produce,so they decided to make it into a animated cartoon to cut costs. After CBS rejected the offer,the studio took it to NBC where it aired for the 1972-1973 season.
However,it premiered on September 9, 1972 as part of NBC's Saturday Morning schedule that produced 13 episodes. It ended on December 2, 1972. Repeats remained at NBC from January of 1973 until its demise on September 1, 1973.