Therefore, its known narrative elements are not a part of the Doctor Who universe as we, on this Wiki, choose to define it. It may have been the basis for a similar story in another medium, however — and that story may indeed be valid.
- You may be looking for the novelisation of the same name.
Doctor Who and the Cave Monsters was a story pitched by Peter Harness for Series 9 of Doctor Who, despite being rejected this story led to Harness being offered to write the two-parter The Zygon Invasion/The Zygon Inversion.
Half a million years ago, some nearby alien race, having made something very toxic with a half-life of billions of years, decided to freight it off into space and bury it underground on a convenient planet. They chose Earth. Having seen that there was the possibility of intelligent life evolving, and being not entirely evil themselves, they buried the stuff deep, and tried to think of how to warn whatever would evolve never to go into the caves. Not wanting to draw undue attention to the caves, they buried their warnings pretty deep.
At the first level, half a mile underground, they put up signs and symbols of death and warning from their own culture. Beyond that, they installed a kind of psychic attack system, which will create the most terrifying monsters out of the psyche of whoever goes down there; in the hope that no-one could persevere when hounded out by their very worst nightmares.
Then they got in their spaceships and buggered off.