If you travel to the surroundings of a town called Pulpí in Spain, you will find an abandoned silver mine. Step in for about 50 meters and enter a strange, shimmering room that you will not find anywhere else on earth.
This unbelievable natural space is the geode of Pulpí that comes closest to the fortress of Superman's loneliness in real life: an amazing egg-shaped cave where jagged shards of brilliant, clear crystal stick out of the walls like teeth in a dragon's mouth.
As far as the geodes are concerned, Pulpís is a giant – one of the largest known geodes in the world.
In terms of the cavern chambers, it is tiny, but big enough for several people to fit in. All at once, which can not be said about most geodesic caves.
Make sure you're careful and trust everyone you go in with: The cavity in the heart of the geode measures In total, only about 11 meters, and with all the sharp crystalline deposits that protrude from the walls, there is definitely no room to slide.  There are more spectacular crystal caves known around the world – especially the famous, towering Naica crystals of Mexico – but how do these remarkable formations emerge?
In the case of Pulpí – which was discovered only 20 years ago – the crystals of Geode's geochemical origins have remained largely unknown, with a background that seems even more mysterious than its cavernous counterparts.
"Uncovering their formation has been a very difficult task, as unlike Naica, where the hydrothermal system is still active, the large geode of Pulpí is a fossil environment," explains geologist and crystallographic expert Juan Manuel García. Ruiz from the University of Granada, the lead author of a new study on the Geode.
In the new investigation, García-Ruiz and his team attempted geological history To reconstruct the geode of Pulpí and to analyze samples from the mineral and geoc hemic environment as well as detailed mapping of the geological structures of the mine surrounding the crystal chamber.
According to the researchers, the geode's gypsum crystals (selenite crystals) grew salt supply through dissolution of anhydrite (the anhydrous form of calcium sulfate) due to a continuous "self-supply mechanism".
This process, which takes place at a temperature of about 20 ° C, was enhanced by a thermodynamic phenomenon called Ostwald ripening (or Ostwald ripening), as well as by temperature fluctuations experienced by the geode at its relatively shallow depth Depth in the mine.
It remains a mystery, however, when exactly this crystal formation took place.
Due to the extreme purity of the crystals in the geode – so clear that you can see through them directly – it is difficult to date the shards, even though the team has some ideas.
"They certainly grew after the desiccation of the Mediterranean 5.6 million years ago," says García-Ruiz.
"That's you." Most likely less than 2 million years old, but over 60,000 years old, as this is the age of the carbonate crust that covers one of the large gypsum crusts [crystals]. "
This is a fairly long time gap that increases the likelihood that other researchers in the future may be trying to narrow the gap.
Until then you can visit the Geode of Pulpí yourself Spanish authorities have opened the site to visitors earlier this year, so now everyone has the opportunity to get into this very strange and special sanctuary.
"Bending your body between the huge crystals is an incredible sensation," the researcher explained Javier Garcia-Guinea, who discovered the formation, the BBC in 2000.
"When I was young, I dreamed of flying, but never in a geode to go inside covered with transparent crystals.
The results are reported in Geology .