Inside the flooded mineral-brine pump galleries of the Pressure Keel, the ballast regulator is failing after years of Reefwalker infestation. A clearance team, guided by passenger-species advisors and watched by a silent forward observer, is sent down to reopen access and restore the regulator before the whole system collapses.

The team begins with standard doctrine: scan the calcified barriers, burn through them, and push toward the lower galleries. But the Reefwalkers do not just occupy the space — their slurry reacts to pressure, hardens into new walls, embeds egg-cysts, blocks extraction routes, and slowly grinds through pressure suits with crystalline dental plates.

This is "The Knock" by Sascha Schmidt.