Air Holes, found below planet Zebes' surface, are artificial pipe-like structures protruding from floors and walls that strongly resemble Warp Pipes from the Super Mario Bros. series. Indeed, in the 1986 manga, Samus mistakes it for one. Like Warp Pipes, they come in a range of colors and a few of them can be entered by Samus. They are only seen in the 2D Metroid games.
Some of these Air Holes, however, have small species of animal emerging from them which immediately attempt to ram Samus by flying towards her. Depending on the region of the planet, Zebs, Geegas, Zebbos, and Gamets can be seen flying out of the artificial structures. Samus can repeatedly kill them for energy and ammunition because these creatures continuously spawn, suggesting they have constructed hives within these tunnels. There are underwater crustaceans called Zoas that only emerge from sandpits found in Maridia instead of Air Holes, but nonetheless fall under the same category as the bioforms mentioned above due to their limitless spawning and attack pattern.
Other than sandpits, it is interesting to note the existence of other naturally formed crevices found throughout Zebes, some of which are filled with either lava or acid, that will also continuously release the "Air Hole-based" enemies.
On planet SR388, there are several corridors containing vine-like tunnels on the floor that infinitely release Gawrons, Skreeks or Yumees, serving exactly the same function as Zebes' Air Holes. In some areas of the planet, these creatures will emerge from pools of hazardous liquid. However, they will cease to fly out from their respective "nests" whenever 10 of these are killed, suggesting the creatures become concerned in preserving their lives rather than continue sending more of their own to face the threat.
There are instances in Metroid Prime and Metroid Prime 3: Corruption where creatures are continuously spawned from pipes or other small crevices, essentially acting as Air Holes. The Puffers, Plated Puffers, Puffer Mines, Phazon Puffers and Phazon Metroids are infinitely spawned within the Phazon Mines Ventilation Shaft, Tower of Light, Airshaft, Drop Shaft and Metroid Cavern, respectively.