A company pushes through an infested refinery town and a hollow drainage ridge, trying to take and hold ground building by building.

The packs are underneath them, breaking through floors, trench walls, drains, and cellars whenever the fighting slows.

Every step becomes a decision. Soft concrete, buried voids, lava-tube seams, and storm drains turn the settlement into a trap with no safe rear line. Comms fail inside the refinery grid, enemy fire pins the assault from the front, and men vanish through openings that were solid ground seconds before.

This is "Bug Swarm Below" by Sascha Schmidt