Inferno was the first story in which producer Barry Letts had any say on development and commissioning. Director Douglas Camfield fell ill after completing parts one and two. This left Letts to direct the other five parts. Houghton had based the drilling part of the story on real events. He had contacted the American Embassy to enquire about Project Mohole, a real world attempt to drill through the Earth's crust. Information on the project and why it was abandoned was not disclosed as such information was top secret. It was this secrecy that Houghton developed upon for his story.