The actor who played Zefram Cochrane in TOS Metamorphosis
, Glenn Corbett, was at one time slated to be in an Irwin Allen production that never got off the ground. It was called "City Beneath the Sea
". The short promo clip I linked shows a scene with Glenn piloting the flying sub from "Voyage to the Bottom of the Sea". It was filmed in 1968, so he looks very much as he did in the TOS episode. Kind of interesting.