People often deride Underworld because of the rampant use of CSO for the scenes set in the planet of the P7E. I find this criticism entirely unfounded. Honestly, the CSO work not only doesn't look bad, it's downright impressive. The scenes are well lit in relation to the model sets, and - oh! - there's even some movement of the background in the scene where The Doctor and Leela are walking down a passageway.
That moving shot is rare, though, by the looks of it, and phenomenally difficult to pull off in the days before CGI, or even SceneSync. As such, there is a static element to many of the scenes, and most of the are shot beginning with the subjects in long shot, then advancing towards the camera during the course of the scene. However, static camera shots are hardly to fault of director Norman Stewart, and Stewart certainly doesn't have a monopoly on them amongst is esteemed peers (I'm looking at you, Peter Moffatt).
It's the tail end of this episode that was probably the first thing I tried to actually sit down and watch when it came to Doctor Who. I was nine years old when I first saw this. I might swimming in nostalgia right now, but I can't help but thoroughly enjoy this story two episodes in.