- Actor-Inspired Element: The scenes of the Doctor shepherding the Trogs through the caves and onto the P7E were devised and organised by Tom Baker himself.
- Follow the Leader: According to the production notes, the recently released Star Wars influenced certain aspects of the production, especially when it was learned that the film would be released in the UK ten days before "Underworld" was scheduled to air.
- No Budget.
- Throw It In!: The scenes of the Doctor shepherding the Trogs through the caves and onto the P7E were devised and organised by Tom Baker himself.
- Troubled Production: The story (and the one that followed) suffered from The BBC slashing the rest of the season's budget due to the economic crisis the UK was suffering, leaving them with barely enough money to build the spaceship set, and nothing to build sets of the caves, or to shoot them on location. As a result, they ended up having to resort to filming all the non-spaceship scenes on a bluescreen, and matte the actors into miniature cave sets.
- Wag the Director: In the original script, Leela was to scream when overwhelmed by the gas in the second episode. Louise Jameson declined to do this, pointing out that it was out of character for Leela.
- Working Title: Underground.
Trivia / Doctor Who S15 E5 "Underworld"