There's living stuff under the ground, who knows what is happening down there, but it's probably supporting ongoing terrorism against the superior surface dwellers!
It's a mod and just a craftable item used in industrial farms if I recall correctly. Although there's a newer mod called Immersive Weathering that has humus and also has some simulated ecological and geological processes.
You can literally take a shovel almost anywhere and find a terror network under your feet!!! We need a scorched earth policy to defeat this dangerous and ubiquitous enemy.