Messinian Salinity Crisis!

Huge crystal cave found in Spain:

Normal-sized geodes rarely extend beyond the palm of one's hand. This one, however, reaches a staggering eight metres long, can fit up to 10 people inside it and is chock-full of prisms.

Javier Garcia-Guinea, the geologist from the Spanish Council for Scientific Research in Madrid who found the cave, can find no other reference anywhere in the world to a geode of such enormous size. The abundant hydrous calcium sulphate deposits, better-known as gypsum crystals, are about a half-metre in length.

Geologists suspect the cave could have formed about six million years ago during the Messinian salinity crisis. During this event the Mediterranean was evaporating, leaving behind massive amounts of salt. The cave may have filled with the same salt water that also would have produced thick salt layers, which, in turn, could have seeded the crystal deposits.