The Mines of Lucania refers to a large resource extraction system in the Roman province of Lucania. More abstractly, the Mines refers to Roman mines in general, of which there were many scattered throughout the Roman republic. Spartacus, Crixus, Mira, Nasir, and a clutch of rebels rescue Naevia from the mines after she is sent there to be worked to death. They later return and liberate the entire mine and free all the slaves, ending the cruel conditions there, saving many lives, recruiting new soldiers to their army, and hampering Roman productivity. The Mines of Lucania are the only mining system shown in the series.
Mines are referenced in general terms many times throughout the series. Dialogue in the series implies that the Mines of Lucania are used to mine salt, though this is dubious. The house of Batiatus sends its failed gladiators and most disobedient slaves there, as do many Roman slave owners. Slaves may also be sent there if they are too old, sick, injured, or uncontrollable to be profitable for their domini. They are generally considered the worst possible assignment for a slave, and being sent there is akin to a death sentence.
Oenomaus and both the elder and younger Batiatus often used the mines as an alternative when gladiators or recruits proved themselves useless or unprofitable, or to threaten them into better behavior; Agron and Duro were threatened with being sent there if they continued disrupting training, Indus was sentenced to the mines during one of Quintus's bad moods, and Dagan and Ashur both faced being sent there at different points during the elder Batiatus's competition.
By definition, mines are subterranean (underground) structures. They are mostly comprised of tunnels carved deep into soil and rock, through which workers travel and extract materials. They are completely dark, with no natural light source. They also have no moving air, no potable water, no food, no restrooms or plumbing, and are often extremely warm, as well as filthy. Additionally, they often have a stale or foul odor, are hives of spreading disease, and can be prone to insurgences of toxic gas, as well as having little oxygen, and experiencing sudden hypoxia (depletion of oxygen).
During the Roman republic, mines were exceptionally dangerous and unpleasant. Laborers in mines were almost always slaves, and were generally treated worse relative to slaves in households or cities. They experienced harsh treatment and abuse, minimal food, many hours of hard labor, and extremely dangerous working conditions. Their life expectancy was considerably lower than most slaves and freemen laborers, usually only a few months at best.
The lack of subterranean engineering technology meant that mine shafts were often unstable, and it was not uncommon for them to collapse with workers trapped inside. Even if they were not crushed by debris or suffocated by lack of oxygen, the fact that mine laborers were almost always slaves meant that they were viewed as expendable: little or no effort would be spent to try and save trapped miners, who would eventually die from thirst, starvation, or lack of oxygen.
Gods of the Arena
While not seen in the Prequel, many gladiators were warned about the mines throughout their training. In Missio, one recruit named Indus is banished to the mines after giving the sign of Missio while training with fellow recruit Dagan during training. Seeing the Missio enflames the insecurities of Batiatus, still reeling from his battering at the hands of Tullius, and views the gladiators' plea for mercy a symbol of his own submission to Tullius. Batiatus decides to refuse Tullius's second offer and not submit to his desires upon seeing Indus give the Missio. He tells Ulpius, the doctore at that time, "Doctore! Send that fucking man to the mines! Do it! We do not surrender in this fucking house! I will see these walls fall to ruin before Missio is given!"
At first all the gladiators are very solemn, as Indus begs Batiatus to change his mind and Doctore to intercede. Doctore cannot even bring himself to look at Indus as the guards drag him away to be chained up. Batiatus remains in a foul mood, and appears to be totally oblivious to the cruelty of his decision and the discord sown by Indus's banishment.
This proves to be a particularly controversial decision, as Indus had done nothing wrong except draw Quintus's ire while in a foul mood. Doctore and the gladiators deemed the order excessive, as being sent to the mines was equivalent to a death sentence, and one of the most feared fates for a gladiator or any slave.
Blood and Sand
Oenomaus believes Spartacus and almost all his recruitment class (excluding Varro) prove that they are not worth being gladiators and should be sent to mines, an observation shared by Batiatus himself.
The mines make an appearance in season two when Spartacus, Crixus, Nasir, Mira, Rhaskos, Liscus, Fortis, Acer, Mannus, Plenus, Tychos, Sophus, Vitus and a handful of Gauls under their command try to sneak in to save Naevia.
At first they are successful, but come under attack by Marcus and Ashur who bring with them dozens of Roman soldiers. This results in the many Gauls' deaths, Crixus, Acer and Rhaskos captured while Spartacus and the others are forced to flee.
During their time there, Spartacus makes mention that they must free all the slaves, but agrees with Crixus that there are too many. As a result, Spartacus vows to return for them when he is able.
War of the Damned
Following Spartacus' victory over the Praetorian armies of Glaber and Varinius, Spartacus and his men assaulted the mines, slaying all the Romans and setting all the slaves free. This sprung their small force into the thousands and ultimately brought in slaves from all over the Republic to join them in rebellion.
As a result, this leaves the mines now empty and, presumably, inoperable until the end of the war when it could be safely reopened.
Historically, there were no salt mines in Roman Italy. However, there were many other types of mines, the footprint of which can still be detected in the environment to this day.
Roman mines were notoriously dangerous. The Roman perception of enslaved people as largely expendable meant that few worker protections existed. Mines were constructed, mined, and maintained with only as much caution as was necessary to keep them functioning and profitable. Many mines had high death rates, requiring a constant supply of new labor. These individuals mostly consisted of condemned criminals and foreign prisoners of war. Lighting in the mines was nonexistent, other than with oil lamps which rested on carved niches in the rock walls. Sometimes these would be candles, and the length of time between being lit and burning out was meant to indicate the beginning and end of a shift, respectively. Mine slaves were known in Latin as Damnati ad Metellum or 'those who are damned to the mines'.
Documentary and archaeological evidence of Roman mining in Italy indicates the extraction was primarily of metals and minerals. The territory called Lucania in the show, located in south-west Italy, now known as Basilicata, was home to many mining activities in Roman times. However, it should be noted that referring to the region as Lucania has become stigmatized and less frequent, as Benito Mussolini attempted to model fascist Italy off of ancient Rome, and encouraged the use of Roman place names like Lucania to show continuity between Roman Italy and fascist Italy. Even the word "fascism" derives from Roman military paraphernalia, specifically the eagle and branches symbol called the fasces. The usage of the name Lucania reminds many of a very painful chapter in modern Italy's past.
Basilicata has long been home to mineral and metal extraction, as the geology is naturally rich in metallic elements and valued minerals and nutrients. Mining persists to this day in Basilicata, though it appears to be mostly petroleum-related materials, with metals as well. The region of Potenza, which the Romans had occupied and called Potentia, still has several active mining operations. Like many Roman mines, although these modern operations are partly-owned and operated locally, investments from the Italian government as well as foreign firms help keep them operating.
As to the idea of Spartacus deciding to liberate one of these mines to swell his ranks, it is not impossible that he did so, but historically Spartacus would have instead gone after the more ready abled slaves such as Pastores (slaves who like Gladiators, were also trained to fight). Liberating one of these mines may have been more appealing on his travel back south after their victory in Mutina in 72 BC, however doing so would have no doubt swelled his own armies followers camp, with only half or less being able to fight.