The Abyss is the Xal-Fettk's underwater base located on a sunken Island in Ga-Metru. Below is information about this infamous base.
The entire base is inside a gigantic air bubble. Thus it is like an actual island, and completely dry. Intrestingly, the bubble cannot be popped, only shrink down over the course of many months, it is sustained by Hydruka that work in a feild of air nearby. Around the rim of the island are many outposts with massive Rhotuka Launchers that cause explosins of ice, freezing much of the water.
Large sparring feilds cover much of the eastern side of the Abyss. These are used by the Tainted to pit her best Agents against newer recruits. However sometimes the tests aren't what they seem, and you could defeat the Tainted herself and still fail.
In the center of the Sparring Grounds is a massive staduim, this is where excutions or other major events are held. The Tainted has been known to fight unloyal Agents here, mutating them after their defeat.
The Fortress is the focal point of all of the Xal-Fettk. Here they house their treasure, they live here, they spend their time when not on missions in this place. It's the only place that most Agents can call home. Where it stands there used to be an ancient castle that the Tainted destroyed in order to make room for her own "castle". As an interesting side note, the Tainted used stolen Antidermis to enslave Matoran to build the fortress for her.
The Throne Room
This is the Tainted's sanctuary. Her throne room is in the middle of the Xal-Fettk's underwater base. It's walls are lined with trophies of her battles, and she keeps an arsenal of her fallen foes' weapons in the room directly next to it. She say's it's the only place she's ever felt safe in, besides the middle of a battle. Whether on not this is true remains to be seen.
The Protodemic Mines
The Protodermic Mines are a large shaft, about 20 Kio around at the largest point, and half a Mio deep. Here the Tainted has many enslaved some nocturnal subterenian Rahi, called Umbrak, working the mines that were found when the Xal-Fettk first drilled the hole. They vary in size and power, but many of them have large pincers used to smash through the hard protodermis. Some have had drills grafted to their arms and use these work more efficiently in the deepest parts of the mine.
The Forges are in the upper part of the mine. Here, many inventors and smiths craft new weapons, armor, vehicles, and the like. To get to the Forge, after you reach about 4 Kio deep, you should see a cave marked "FORGES", follow that and you will eventually reach the main Forge with a large furnace in the center.
The only people ever to come out of here alive are Loraska and the Tainted. No one is quite sure what's in there, all they know is this is where Loraska and the Tainted hide all of their most valued treasures. Rumours of this place being filled with wicked Rahi abound. Some say that the journey underwater turned them into freaks, others that Karzahni himself put evil creatures in this place. Thus coining it's name. Whatever the truth is Loraska and the Tainted refuse to say anything about it, and people very much doubt it's because they beleive that knowledge is power.