Among the numerous horror movies I watched this past October this one is a favorite of mine: X The Unknown, an early Hammer produced sci-fi/horror film from 1956. In this movie a menacing, highly radioactive and seemingly intelligent blob of surging mud oozes out of a crack in the ground and begins killing locals as it searches for "food", in this case energy sources. The malevolent muddy ooze has a decided preference for radiation and it enlarges as the movie progresses. There's a final showdown between the monstrous ooze and a group of scientists and military who have united to stop it. (no other spoilers).
The DVD for this early Hammer horror movie can be purchased online at Amazon.