The dark void that houses the fifth floor's bubble-worlds — an egg-carton lattice of sunken domes over a hidden floor beneath the planet's surface.
The Lacuna is the fifth floor's true structure — the black, cold, red-tinged void that holds all the bubble-worlds, revealed once Carl flies out of popped bubble 543. Far from a flat sheet of bubble wrap, it's more like an egg carton: each self-contained snow-globe world sunk into its own socket, half of them popped and half intact and glowing like frosted lightbulbs, with the 'land' of the Lacuna only a few hundred feet below each quadrant's waterline. Being under the planet's surface, it has an unseen roof overhead to avoid.
Across this void Orthrus bounds after Carl's plane, leaping bubble to bubble toward the burning glow on the horizon that is the god Emberus.
A dark, cold, red-tinged void filled with sunken glowing and popped bubble-domes in an egg-carton lattice, a burning god-glow on one horizon, an unseen ceiling above.
The void housing the floor's bubble-worlds, traversed in the aerial dog-chase.