Pozos de Hathsin
The Pits of Hathsin are were the Lord Ruler sends rebellious skaa in order to punish them. No skaa return from the Pits alive. They are also the only place where atium crystals can be found. Allomancy cannot be used around the crystals, as they will shatter. That is why skaa are used. All shaa in the Pits have small scratches along their arms from the 'teeth' of the crystals. The Pits are looked after by House Venture.
The Pits are seen as Hell for many skaa. Each skaa has a week to find an atium geode. If they fail, they are beaten savagely to death.
Kelsier is the only known person to survive the pits. Only days before the fall of the Lord Ruler, Kelsier returned to the Pits, destroying all the atium crystals he could find, and setting all the skaa prisoners free.
Brandon has also confirmed that the Pits of Hathsin is the resting place of the liquid form of Ruin. Prior to the Ascension this metallic lake resided in the Terris mountains, underneath the Well of Ascension, the liquid form of Preservation.