A 100-Swarmer Horde inside the Ice Wind Lava Caves that you must clear at the end of What It Takes To Survive. It does not count toward Horde Killer, so treat it as a story fight in tight tunnels.
The lava tubes are narrow, which helps you. Back into a tunnel so the Horde can only reach you a few at a time. Fire spreads well in the tight space, so throw Molotovs where they pile up.
You come here to meet O'Brian and then track Cooper through the caves. O'Brian explains that Hordes hold together by scent, and that the Freakers are changing fast.