The Atria are the massive galleries that comprise the the habitable, well-mapped regions of Infra. The Great Stairway and the steppes surrounding it are contained in a single, gigantic atrium; the Drow Empire spans two (or three, depending on whom you ask) atria of varying size; and the Thri-kreen honeycomb is sequestered at the far end of an atrium shared with shadar-kai and exile vagrants, gnollish brigans, and more.
The Atria contain about every kind of natural wonder one might hope to see — pristine, clear-water streams filtered by the miles of ground above the heads of the Infran prisoners; rivers that rush like the rapids on the surface; whole galleries that glitter with any light introduced to them for the covering of crystals they have; pockets so large from floor to ceiling that nearly mountainous stalagmites and stalactites rise up from the floor and hang from the roof, respectively. They also boast the oddest assortment, small numbered though it may be, of flora and fauna. Mushroom-esque trees that grow to two times the size of a man or elf, anemone-like corals and carnivorous plants, huge, beast-of-burden-sized lizards and rats.
It is not uncommon to see animals the surface fears and drives away hiding in the Atria of Infra. They, like those who have grown up in the confines of the World-Beneath-the-World, seek a place of safety.
The Atria are astoundingly huge, with some galleries having not even been properly surveyed for colonization.