The Quatermass Xperiment

The Quatermass Xperiment, released in the U.S. as The Creeping Unknown, is a 1955 science fiction film directed by Val Guest and starring Brian Donlevy. The score was composed by James Bernard, who wrote the music for many horror movies.

I love the ending of this one.

LiveVideo has this one online here, but I can't get it to embed properly on my blog.

Moria's review says that

The Quatermass Xperiment was a genre cornerstone in a number of crucial areas. It was one of the earliest productions made by Hammer Films – and their first international success.
...
Equally importantly The Quatermass Xperiment was – with the exception of the cheesy Devil Girl from Mars (1954) – the first British film to venture into the theme of alien invasion that was running riot across the Atlantic

1000 Misspent Hours calls it "probably the most important horror or sci-fi movie ever made in England" and closes with this:

The message seems to be that hard times call for hard men, and though such people are profoundly dangerous to have around, we'd be even worse off without them. It's a rather more sophisticated perspective than one usually finds in movies that deal with mad or half-mad science, and in the context of the 1950's, it's probably also the most honest assessment of the role the military-scientific-industrial complex plays in modern Western civilization.

SciFi.com has a review. The BBC has a celebration of the original and the remake. This site has a couple of quotes from film/science fiction encyclopedias.