At the Earth's Core (1976)

A Victorian scientist and his young American backer set off in their new earth-boring machine with a short test on a Welsh mountain. Unfortunately the thing is rather more powerful than expected and they end up in an enormous cavern at the centre of the earth. Here they find a race of humans enslaved to evil oversized prehistoric birds with extra-sensory abilities. The travellers decide they can be some help, the more so as at least one of the slaves is very eye-catching.