The Crystal Cave in Almeria

The Crystal Cave in Almeria was discovered by a Spanish geologist in an abandoned silver mine in 2000. The find was extraordinary featuring some of the biggest geodes ever recorded. A geode is a hollow crystal lined rock. A typical geode would measure about the breadth of your hand, but the geodes discovered in the cave were an enormous eight metres long. The Crystal Cave measured 1.8 metres wide, 1.7 metres high and 8 metres long and was entirely covered with gypsum crystals.  
Some of the crystals of gypsum were measured at over a half a metre in length. They were described as being "absolutely transparent and perfect". At the time, police were placed to protect the cave from looters trying to get a souvenir of the beautiful crystals. Javier Garcia-Guinea, the Spanish geologist who discovered the cave hoped to study the cave and then eventually open it up as a tourist attraction.  It was thought that the crystals may have been formed 6 million years ago during the Messinian salinity crisis. This was at a time when the Mediterranean Sea evaporated, leaving thick layers of salt behind. These deposits could have filled the cave making the geodes like they are today.