After having just watched the reprise from Episode Two, I am absolutely blown away by how good the sequence looks. The Doctor is fighting through a steady haze of fumigating gas, trying to find the shutoff valve. The scene features Tom Baker, stumbling through a set consisting entirely of blue screen, as the camera follows him over to the shutoff valve, while another camera (live, with no computer assistance or duplication), and all the while, a fluid, smoky haze is superimposed over top of the whole picture.
Remember - this was broadcast in 1978, and probably made shortly after Star Wars had turned the world of science fiction on its head. And even then, Star Wars featured next to no blue screen work with live actors, as the technique was most often used exclusively through model shots. The effects seen in Underworld are staggeringly good not necessarily because they are actually good, but because they should be much, much worse.
Oh, I like that dreamy little but of music whenever The Doctor, Leela, and Edas float down the core of the planet. Simpson's scores were starting to become a little bit tired as the years went on during his (by now, non-stop) tenure on the show, but it's good to see he can still come up with something memorable. The music during the sacrifice scenes in the P7E are equally good.
Cool - lots of laser gunfights in the latter part of this episode. I can see why my nine-year-old self was becoming hooked on this show.