At the Earth's Core (1976)

Dr Perry (Peter Cushing) and David (Doug McClure) head into solid rock using a giant tunnelling machine. The machine goes slightly wrong and they end up marooned in a strange new tropical world deep underground instead of Wales. It is a strange world inhabited by dinosaurs and girls in skimpy ragged outfits (so probably not Wales then). Perry and David end up captured and sent as slaves to the sinister Mahar, avian dinosaurs who control their underlings with psychic powers...

David fights his way to freedom, linking up with tribal leader Ra (Cy Grant) and falling for the lovely Dia (Catharine Munro). He and Perry unite the disparate human tribes to fight and destroy the Mahar. Then they can repair their tunnelling machine and head home happily for tea...

And it is all wonderful tosh with questionable (though at times pretty decent) special effects, cliched fights with beasts and weird monsters. Don't ask how the humans in this underground world can speak English though, it remains unexplained like much of the film!