In the Demon Spawn War, Siolix was a mining area controlled by the demon lord Zuggtmoy. It was not worked by the demons or any of her soldiers because early mining efforts led to many deaths from demons mutating into raging, uncontrollable and insane monstrosities. This is because the area is laden with veins of deep rock. Her adviser in charge of production decided to use slaves to tap the area's rich veins of gemstone, copper, and cassiterite (source for tin).
The mines were a deadly place for many, for once you had the sickness it could not be removed short of the skills of a master healer. The lucky survived and mutated, the unlucky became food for her green slimes, black puddings, or gray oozes.
- Borri, from his book History of Me - "The Mines of Siolix"
The earth giants had the best survival rates in the mines, partially from their fortitude and the favor of Geb. Those that did survive, mutated into what has become the first fomorians. After the Demon Spawn War, Geb had the entire area sealed off. There are three entrances to the place, each a fortress garrisoned by huge earth elementals and other earthen loyal to Geb. The earth elementals periodically move through the rock surrounding the area, making sure nothing has dug in. In addition to the earthen is a channel of the Earth Seam which was redirected by Geb to form a continuous defensive loop around the entirety of Siolix. The two Earth Seam sinkholes of the area are connected to each other, so this part of the Earth Seam cannot be used to reach other places of the Underdark.
Siolix is a haven for puddings, slimes and oozes. They feed upon each other, but given the chance they will leave the overcrowded conditions and get a taste of what is on the outside.