The year was 1967. Irwin Allen had gotten renewals for Lost in Space & Voyage to the Bottom of the Sea, which would enter their final seasons that fall. Time Tunnel, however, was cancelled after 1 season at ABC, and Land of the Giants, intended for the 1967-8 season, was delayed a year, where it would eventually replace Voyage on ABC's schedule.
NBC, understandably, wanted a piece of the action. Network VP Herb Schlosser, showing little or no faith in Star Trek, commissioned Allen to produce a series for the Peacock Network, meant to replace Trek. Of course, as we all know, Trek had already become a fan favorite despite being a little challenged in the ratings, and would forge ahead for 2 more seasons.
City Beneath The Sea wound up being passed over when Trek was renewed. Undaunted, Allen would try again 4 years later, but with a different cast. In addition, he took City to WB for his 2nd attempt, which at least saw the pilot film air, but no series.
The 1967 version is headlined by Glenn Corbett (ex-Route 66) and James Brolin. Lloyd Bochner was posited as the villain of the piece in much the same way Jonathan Harris was as Dr. Smith on Lost in Space 2 years earlier, and Kurt Kasznar would have that same distinction in Giants.
The fight scene between Corbett & Bochner, or at least their stunt doubles, resembled a generic Trek fight scene (the monkey flip spot is the dead giveaway) in its concept. The basic premise, though, would tip the direction Allen would take with his disaster movies in the 70's.
Surely, NBC could've found room for both City and Trek, but didn't. Something tells me Schlosser was looking at profit margins, thinking City wouldn't cost as much to make as Trek, but I can't be certain of that at all.