The episode: “The Phantom Planet,” ep. 902
The riff: Narrated by Mike as the astronaut hero of the movie steps out of his spaceship onto the Martian surface, with stone columns on either side.
The explanation: The columns hemming him in are the important part of the reference here. In Greek mythology, Scylla and Charybdis were a pair of nigh-unavoidable obstacles that threatened sailors between Sicily and the Italian mainland. Scylla was a six-headed monster that ate passing sailors, while Charybdis was an ever-churning whirlpool that would suck entire ships down. The pair were located next to each other, necessitating a choice of which danger to face. In Homer’s Odyssey, Odysseus chooses to face Scylla and lose fewer men in the passage. In modern times, they’ve come to represent difficult passages through a challenging situation.
Novelty factor: It probably comes to no shock to you at this point that I know Greek mythology pretty well. Back in my high school scholastic bowl days, these kinds of categories were my bread and butter. As a riff, it’s just another example of the MST3k writers’ versatility.