This early Hammer movie is very similar to their QUATERMASS films. British soldiers, doing exercises in how to detect radioactivity, detect that something else, not their own sample, is emitting energy. Two soldiers are contaminated and have radiation burns during a strange, localized earthquake. Then a kid in the nearby woods sees something horrendous, which gives him radiation burns and eventually kills him. A philandering doctor is literally melted to death by something that crawls into his x-ray lab. Whatever the creature is, it feeds on radioactivity.
An American scientist, already doing experiments in the UK, is called to investigate and believes it probably originated at the first test area. It came from deep within the earth. But by the time they get back to that spot the two posted guards are killed. And the creature is on the prowl, a sort of giant blob, which is exactly what it looks like. I think this movie works so well because it's totally serious and plausible, unlike the majority of the US radioactive monster movies that came out during that same time period.