Almadén is home to the world's greatest reserves of cinnabar, a mineral from which mercury is extracted. Cinnabar was first used for pigment by the Romans. Later, the mineral was used mostly in medicine and alchemy during the Arab domination of Spain.
The Fuggers of Augsburg, two German bankers, administered the mines during the 16th and 17th centuries in return for loans to the Spanish government. Mercury became very valuable in the Americas in the mid 16th century due to the introduction of amalgamation, a process that uses mercury to extract the metals from gold and silver ore. The demand for mercury grew, and so did the town's importance as a center of mining and industry. Most of the mercury produced at this time was sent to Seville, then to the Americas.
The dangerous working conditions of the mines made it difficult for the Fuggers to find willing laborers. As the demand for mercury grew, the idea of convict labor was introduced.
Some photos here.
There are scores of photos of the area on GE.