The porthole was showing lighter rock.
Not dramatically lighter — just the specific quality of stone that had been compressed sand before it was compressed stone, the geological memory of a desert that had been doing something else before the world ended and fused it into this. Mojave bedrock rather than continental crust. The color of old ash rather than old dark.
Will noticed it from the engine room corridor. He'd been watching the pressure gauge for forty minutes — the same cracked gauge, the same worsening numbers — and the rock color change was the first thing that had happened in those forty minutes that wasn't the gauge telling him something he already knew.
He went back to the cabin.
The floor was tilting.
