Inside the subsurface mining galleries of a fractured, freezing moon, the 3rd Section of the 77th Expeditionary Engineer Battalion descends through unstable rock, dead machinery, ammonia-argon atmosphere, and broken corporate maps. Their mission is to carry a data core to the lower relay chamber and transmit a warning before the buried threat spreads farther.

The threat below is the Choir: a distributed biofilm intelligence that converts biomass into military infrastructure, absorbs tactical knowledge from consumed hosts, and uses dead soldiers as active signal relays. As the engineers scan walls, cut through locks, patch failing suits, and force their way through service drifts, the organism learns their tools, their formations, and their frequencies.

This is "The Singing Deep" by Sascha Schmidt