The carvings, which are growing more "decadent" and debased, continue to tell the Old Ones' story. Climate change and dwindling resources drove the Old Ones underground.
The carvings reveal the presence of a vast subterranean sea beneath the city, where the Old Ones built a refuge. But that's that all: not far away lie mountains that dwarf those that shield the city.
And these mountains are the home of something too horrible for the Old Ones to even depict. Dyer and Danforth continue their descent into the unknown.