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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.
YES! I watched it on bootleg VHS about 100 times when I was a kid. It has so much to love.
*myrmecologists *a lady scientist *flamethrowers and gas bombs *LA sewer tunnels *some psych ward patients who make some great points (eg - Do you think there's a black market for hot sugar? They were ants, I tell you!) *walk by cameo by a young Leonard Nemoy; he was just an extra at that point *an entire informational film strip about ants
They also do a really good job of investigating and escalating the emergency response to a very bad and weird situation. Good writing and good acting.