Ian also saw the air purification units.
Every fifty meters, a waist-high Metal Box was embedded in the wall, its surface covered with pipes and filters.
A faint hum emanated from inside the boxes. They drew in the foul air from the surface, and after multiple layers of filtration, expelled gas that was barely breathable.
The filtration efficiency was low. The purified air still had a pale purple tint, its toxicity merely reduced to a level that wasn't immediately fatal—but it was enough for humans to scrape by.
At a depth of one thousand meters, the tunnel ended at a heavy metal blast door.
The door was cast from some kind of alloy, its surface covered in scratches and dents from impacts. Twisted Runes were drawn along its edges.
Ian glanced at it and could roughly tell it was a crude ritual to ward off filth.
Faint human sounds came from behind the door: suppressed conversations, low coughs, and the weak cry of an infant.
