The Pit

The Pit was an underwater jail used by the Order of Mata Nui to imprison those who were considered unredeemable. The Pit had a jailer (Hydraxon) and two Pit Guards (Maxilos and Spinax). The being who brought the prisoners there was the Order member Botar. However, the jailer was killed by the Barraki Takadox during the Great Cataclysm and Maxilos was possessed by Teridax 1,000 years later. The Ignika eventually recreated the Matoran Dekar as a new Hydraxon with no memory of dying.

Post-Great Cataclysm
In the Great Cataclysm, the jail part of the Pit (referred to as the Inner Pit) collapsed and flooded. Many prisoners, such as Toa Tuyet(the one from a different universe) died, but several escaped and were turned into water-breathers by the Mutagen in the waters of the Pit. Zyglak began to inhabit the inner Pit after the Great Cataclysm.

Geography
The seas around the Pit, referred to as the outer Pit, were black and filled with danger. Here, the six Barraki lived after escaping, along with many other mutated former prisoners wandering along its seabeds. Ehlek also built a base there, although it was destroyed when Mahri Nui sank.

Notable Features

 * The Pillars of Salt was an area at the northern edge of Mahri Nui, which served as Pridak's favorite hunting grounds.
 * The Razor Whale's Teeth was an underwater rock formation in the exact center of Barraki territory.
 * The Octo Cave was a cavern where Kalmah first found the Sea Squids. He banned the other Barraki from accessing it after Mantax ate half the brood.

Mutagen
The waters of the outer Pit contained a strange Mutagen. It transformed almost anyone who stayed in it long enough into Rahi-like water-breathers, including most of the prisoners of the Pit.

Known Prisoners

 * The Barraki
 * Pridak
 * Kalmah
 * Ehlek
 * Takadox
 * Mantax
 * Carapar
 * Nocturn
 * Tuyet (From another dimension)
 * Brutaka
 * the Matoran leaders of the Metru Nui Civil War (though it is unknown if they were still in the Pit during the Great Cataclysm)
 * At least one Skakdi. He was wounded by Pridak and left for the ocean predators.

Non-Prisoner Inhabitants
These beings inhabited the Pit, but weren't prisoners.
 * Toa Mahri
 * Jaller
 * Kongu
 * Hahli
 * Nuparu
 * Matoro
 * Hewkii
 * Hydraxon
 * Maxilos
 * Teridax (while controlling Maxilos)
 * Spinax
 * Lesovikk
 * Karzahni
 * Zyglak
 * Mahri Nui Matoran
 * Gadunka
 * Several types of marine Rahi

Trivia

 * Mantax searched The Pit every night for something that was apparently important to all the Barraki, though none of them knew what it was. This thing had a symbolic meaning and was made of stone. It also had something to do with the Brotherhood of Makuta, and may have been a contract of some kind. It was confirmed that this is the Betrayal Contract.
 * Before the Inner Pit was flooded and destroyed, The Pit was a cave with cells.
 * It is now known that the Zyglak roamed the remains of the old Pit, for nothing else really lived down there after the Great Cataclysm.