Whole buildings seems to have sunk down into the Underworld and become part of it. Deeper down, the tunnels become less and less traversable for humans, filled with detritus, strange growths and hinders. Nobody has ever seen the roots of the City. The Monuments are strange. They are constructions which seems to retain their place and shape regardless of time.
Normal buildings flourish, die and are replaced about every millennia.
But the monument buildings do not change, no new buildings try to reshape them and they do not change form. Most are guarded by powerful gods or spirits, and often important places. A typical example is the Temples of the Dead, where the dead are received by the priests and sent down into the Underworld through the White Elevators. Another typical monument is the Pillar of Time, a transparent pillar several meters high. On its surface, a set of thin red lines imperceptibly creeps towards the apex over the span of several years, then turns the pillar opaque red for a few minutes, and then it clears, and the lines start to rise again.