In 1910, during the mining works, what was then called the Cueva de las Espadas (Swords Cave) was discovered; this was a single 80 m diameter great hall 120 m below the surface. Its basic feature was being filled with big gypsum "prismatic" crystals (gypsum macrocrystals are generally called "selenite") reaching 2 m in length and 25 cm in diameter, that underwent mineralogical exploitation. As a matter of fact most macrocrystals of this kind exposed at the present time in the mineralogical museums of the world come precisely from this cave.
This exploitation has damaged the cave in two different ways: it deprived it of its best pieces on one side, and radically modified the microclimate features on the other. As a consequence, the remaining crystals have turned dusty and opaque.
In April 2000 the excavation of a communication tunnel three hundred meters below the surface unveiled the presence of the Cueva de los Cristales (Crystals cave) containing the world's biggest and purest gypsum crystals ever found, reaching up to 10 m in length and 2 m in diameter.
(deduct 1 meter from max length for faceting and polishing)
A geode 80m across! 
