Spoilers

The seal and the blight

Ordinary mining cannot cut through the seal at the bottom of the world. Select it and use Breach to order that work. The miner needs a place to stand beside it, and the order can be cancelled in Work before it finishes.

An open breach raises the Clamour, sends creatures towards the colony and spreads grey blight across known walkable floors and stairs. Resealing needs eight iron ingots and sixteen masonry blocks in connected storage, then builders must carry them down and finish the job.

Resealing stops further creatures emerging and stops the spread. Blight already on the floor stays until it is scoured. Scour blight uses salt. Shrines protect nearby floor from new spread, but do not replace cleaning existing blight. Blight sickness comes from standing on that grey floor. Its treatment uses water and salt, and the patient needs to stay off the blight.