For the special features of the official DVD release of the story, cast and crew look back at the making of Underworld: Part One (1978).
Into the Unknown is a retrospective documentary which goes into great technical detail about the making of Underworld: Part One (1978), one of the most ambitious productions of the "classic" run of the series and one of the very first television dramas made in the UK to use extensive chroma key in place of real sets.—Anonymous