Nigel Kneale's Quatermass TV series spawned a brief film series produced over an eleven-year period; 1967's Quatermass and the Pit, released in the US as Five Million Years to Earth, was the third and (until 1979's Quatermass Conclusion) last. As in previous chapters in the Kneale saga, the film begins with a baffling scientific discovery.
This time it's a bunch of prehistoric skulls, discovered during a subway excavation in the heart of London. Sequestered in a lab, the skulls start to emanate a bizarre force over the populace, resulting in death and destruction.
Professor Quatermass (Andrew Keir) concludes that the skulls are the residue of an extraterrestrial invading army -- a theory which (as usual) is scoffed at by the authorities until it's almost too late.
Blessed with superb special effects and an expertly sustained tension level, Quatermass and the Pit is easily the best of the short-lived Quatermass series.