Quatermass and the Pit (1967), directed by Roy Ward Baker.
First review
This is the one people seem to remember. The first color Quatermass is a Hammer remake of a 1958 TV series of the same name. Aka Five Million Years to Earth.
Construction workers discover fossilized human remains under London. They then uncover a strange object and call the bomb squad (always the cavalry in British pictures, for obvious reasons). It turns out to be a Martian spacecraft millions of years old. As it comes alive we see strange phenomena and people start changing. The area has always been a spooky, paranormal hot spot, with strong suggestions of the diabolic. The combination of straight science fiction with the dread of ancient evil builds to a dynamite climax.
Quatermass is played differently here: he's honestly afraid of what he discovers and is vulnerable to Martian mind control. As always, and fortunately for us, the Martians left a hole in their defenses that the feeble earthlings can exploit.
Slow buildup as the clues accumulate but that is part of the fun when the audience is ahead of the characters ("Don't go down there, fool!"), although the squabbling bits with the bureaucrats tend to drag. We have a pretty female scientist but no romance subplot.
Something that struck me when I first saw this on TV as a boy were the closing credits, which are presented against the background of two survivors in the wreckage. They are just staring, gasping and trying to recover, but otherwise doing nothing. At the time it seemed to me that this was meant as an ominous postscript, that after the Martian influence was destroyed the human mind was also deactivated to some degree. But now I'm not sure.
Available on region 2 PAL DVD. The region 1 NTSC versions seem to be out of print and are expensive on the used market.
Second review
A few additional notes and new thumbnails from the Blu-ray.
The Blu-ray makes it a much richer-looking film than I remember.
Quatermass is idealistic here, leaking details of the find to the newspapers. He and the government are not at all happy with each other.
I'm not sure I understood the plot when young. Millions of years ago insectoid Martians visited Earth and took away ape-like proto-humans, performed genetic modification on them and returned them to Earth again. They became ancestors of modern humans, at least in part. When the Martian influence grows strong the insectoid behavior emerges, with violent racial purges of all who are not of the pure hive. The latent alien influence is responsible for violent human behavior throughout history.
Tobe Hooper's Lifeforce (1985) is a similar scenario, with space vampires instead of Martian insects.
A shout-out to top-billed James Donald as a hero scientist. He was a familiar face in POW films: King Rat (1965), The Bridge on the River Kwai (1957) and The Great Escape (1963).
Hammer regular Barbara Shelley was last seen in Village of the Damned (1960). I did not know she had red hair!
Available on Blu-ray from Shout Factory with three commentary tracks.