I think I remember that episode. Oh wow, that would have been 40 years ago and that show was old even then! Somehow the Doctor landed on an alternative earth, where they were experimenting with drilling deep into the earth. That destroyed the alternative earth. The Doctor managed to escape back to our earth and warn people not to drill too deep into the earth. He stopped a project just in time on our earth that destroyed the alternative earth.
Correct in every way.
Inferno shows up a lot on the classic Dr Who channel on Roku.