The Underwater Menace, originally called Under the Sea, was Geoffrey Orme's second Doctor Who script submission - his first, The Evil Eye, having been rejected.
Orme was a veteran script writer, having worked on films and television since the 1930s. Before Doctor Who, he'd also written episodes of The Avengers and Ivanhoe. Other working titles for this serial were Atlanta and The Fish People.
Director for the serial was Julia Smith, previously responsible for The Smugglers. Julia is perhaps best known for co-creating Eastenders for the BBC. She also produced Eldorado, but we won't hold it against her...
Location work for the volcanic island was undertaken with the series' regulars at the charmingly named Winspit Caverns in Dorset. The production team then headed for Ealing to film various Fish People scenes and Zaroff's demise.
The story was the last to feature the Second Doctor's stove pipe hat, although it did make a cameo in the BBCi webcast of Shada. Again the master of disguise, Patrick Troughton dressed up as a gypsy in episode three.
Scenes of Polly undergoing an operation to become a Fish Person drew complaints from the National Society for the Welfare of Children in Hospitals, who felt they might frighten children due to have operations of their own.
Contrary to popular myth, actor Joseph ('Nothing in ze world can stop me now!') Furst didn't put on a ridiculous accent to play Zaroff: he was using his normal speaking voice.
The only surviving episode, part three - plus some brief clips rescued from the vaults of the Australian TV censors - was released as part of Doctor Who: The Missing Years, a tape that formed part of BBC Video's The Ice Warriors Collection.