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Them! (1954)
(feddit.nl)
A HORROR HORDE OF CRAWL-AND-CRUSH GIANTS CLAWING OUT OF THE EARTH FROM MILE-DEEP CATACOMBS!
The earliest atomic tests in New Mexico cause common ants to mutate into giant man-eating monsters that threaten civilization.
The slow burn on introducing the ant was awesome. Everyone complains about the smell, not knowing it's formic acid and a tell tale sign of ants, and slowly the answer is revealed but it's unbelievable by most. It was that tension from the dramatic irony that helped make Them! the defining movie of the giant creature genre. Such a great movie.
If you like Them! check out Tarantula.