The Cobalt Mines of Shokan is a location in Outworld which is often used as a prison for "the lowest scum of all realms" by Shao Kahn and the Shokan race (the latter is the reason why they are named as such). Unlike real-world cobalt mines, these mines contain pure cobalt embedded into the rock walls of the mines. The cobalt is blue in color rather than silver.
The cobalt mines themselves seem to be an original creation for the show and comic book, and so far have made no official appearances in any of the games. In Undying Dream, the Mines are revealed to be "as big as some realms" and have at least one-hundred countable levels. It is also referred to as a "Cobalt Prison" and a "blue hell" by various characters.
In the 1998 TV series Mortal Kombat: Conquest, Shang Tsung is imprisoned here by Shao Kahn after his defeat at the hands of the Great Kung Lao; here, he kept the Kreeyan princess Vorpax, who was also a prisoner in the Mines for lying to others and to herself, as his personal slave. Taja was manipulated by the Kreeyan princess into thinking that her long-deceased father was still alive and was a prisoner in the Mines, only for it to all turn out to be nothing more than a trap for the Great Kung Lao.
The mines contain a execution/torture chamber (showing several emaciated male prisoners chained by their wrists; at the end of the chamber is a wrought-iron sarcophagus containing a worm-infested rotting skeleton). Shang Tsung's powers are unaffected by the cobalt in the mines, thanks to his human heritage, but in everyone else's case, the cobalt poisons them or weakens them (such as Quan Chi).
At the end of the series, Taja's friend Dion later was sacrificed to open a portal to Outworld permanently and she is banished to the Mines as punishment.
In Other Media
In the Mortal Kombat 4 Limited Edition comic, Reptile is banished to the mines by Queen Sindel for the crime of genocide committed under his service to Shao Kahn. However, Reptile's bio in the Mortal Kombat 4 game itself states that he was instead banished to the Netherrealm.
Food provided to most prisoners in the Mines is some kind of soup or stew served in a clay bowl, but Shang Tsung eats something that seems to resemble shrimp in the episode Undying Dream.
Some miners look like ragged, but muscular long-haired men, while others are dressed in dark cloaks or ragged clothes. Prisoners are dragged to lower levels of the Mines in chains, being led by guards that seem to resemble reapers (as they hold scythes). Some prisoners can be heard screaming in pain as they are tortured; in at least a few episodes, what sounds like a female's scream can be heard, suggesting that none of the guards discriminate prisoners based on gender.