The episode: “Colossus and the Headhunters,” ep. 605
The riff: Sung by all three of the crew in the theater as a tower of smoke rises from a fire on the ocean.
The explanation: The guys are paraphrasing lyrics from country singer Johnny Horton’s 1960 hit “Sink the Bismarck.” Interestingly, these are not really that close to the song’s actual lyrics, which go “We found the mighty Bismarck, and then we cut her down.” As often seems to be the case with MST3k riffs, it seems they just recalled the lyrics from memory and inserted them without checking to make sure everything was correct. This is probably the kind of thing you wouldn’t see anymore in the internet age.
Novelty factor: I’ve heard of this song before, along with the film of the same name, and I know who Johnny Horton is, but I had never actually heard it until now.