Firelight, stone, and the ordinary business of a home.
The cobblestone holds the day's heat until well after dark, and the first thing anyone notices, coming down from the surface, is that it never truly goes cold. Hearths sit in every third doorway, banked low and never let to die, and the smoke finds its way up through vents worn smooth by generations of the same rising column. Someone is always cooking. The smell of woodsmoke and root stew moves through the tunnels the way weather moves outside.
Rope bridges cross the gaps between the upper ledges, strung with lanterns that burn a low amber and, here and there, a smaller point of colored light: an ember charm hung over a doorway, a hand cupped briefly around something that glows. Children run the rafters before anyone can talk them out of it. Laundry lines cross overhead between the rooftops, and past the last lantern, out where the torchlight gives out, the space keeps climbing into a haze nobody has ever mapped the top of.
There is no machine sound anywhere in it. Every door, every bracket, every ladder rung is hand cut and worn smooth by hands, not by tools running on anything but muscle. It is loud with voices and quiet in every other way a home is quiet: no engine idling, no static, nothing electric humming behind a wall.