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An underground jewel

The geode measures eight metres long by 1.8 wide and 1.7 high.
The geode measures eight metres long by 1.8 wide and 1.7 high. / AYUNTAMIENTO DE PULPÍ/TECMINSA
  • The gigantic geode in Pulpí, Almeria, has opened to the public for the first time this week. The mine was worked for a hundred years, but it took an amateur collector to discover its hidden gem

A mine may have been abandoned by its miners, but it is still of interest to people who like to search for small minerals, either for their own collections or to sell on specialist websites or at fairs and street markets.

The giant Pulpí geode, the biggest in Europe, is in the shape of an elongated rugby ball and measures eight metres long by 1.8 wide and 1.7 high. Its crystals, which resemble knives of ice up to two metres long, are so astonishingly clear that it is possible to read a book through some of them. In the whole world, only one larger than this has been documented, and that is in the Naica Crystals Cave in the Mexican state of Chihuahua.

A geode is the result of a rare combination of accidents and the meticulous work of nature, says García, who knows these underground treasures very well. For one to be formed, a rock cavity fills with hot water with dissolved minerals, which then slowly cools. They have different colours and features depending on the type of materials and the conditions in which they form.

In the case of Almeria, the rock is a hollow dolomite in which a fissure appeared, through which the liquid with calcium sulphate (gypsum) of volcanic origin entered, flooding the cavity during tens or hundreds of thousands of years. It is hard to determine when it occurred, but Juan Manuel García calculates that it was between two million and 90,000 years ago. The oscillations of temperature in the Earth's crust during millennia in the Sierra de Aguilón meant that in the interior the smallest crystals dissolved and the biggest ones grew extremely slowly.

The experts don't know how many of these almost magical capsules exist in the Earth's crust but, given that they are found at a certain depth, it is normal for them to be discovered during excavations to extract minerals for commercial use.

The Pulpí geode is exceptional in that way, because it was never discovered before even though iron, lead and silver were extracted from the Mina Rica from the end of the 19th century till 1970. It was very lucky, because at that time there was not much sensitivity towards these geological miracles: the picks and explosives crashed through everything. In fact, in the mine there are the remains of half a dozen other, smaller, geodes which were split by the excavation and revealed their interior treasure. The workers most certainly didn't think they were important and used to take 'diamonds' home as a pretty souvenir, without thinking about the fact that they were the result of millions of years of intimate contact between the stone and water.

A secret revealed

After the accidental discovery by a collector, a group of local researchers studied the find and notified the relevant authorities to try to guarantee the protection of an extremely fragile treasure. However much they tried to keep it a secret, though, rumours spread among mineralogists and they couldn't stop two crystals being stolen before the cavity was sealed and monitored to prevent the curious and looters entering. During these 20 years, the Junta de Andalucía has still not managed to complete the process of classifying the geode as a piece of geological or natural heritage.

Pulpí council has always wanted to include the remains of its mining past in the Pilar de Jaravía district, and especially the geode, on its list of tourist attractions which currently focuses on its ten kilometres of beaches, golf and agro-tourism. Since 2015, visitors at the San Juan de los Terreros castle on the coast, three kilometres from the access to the mine, have been able to take a type of tour by putting on 3-D virtual reality glasses.

Now the council has taken a step further and, with help from the Almeria provincial government, has invested half a million euros on preparing the site for tourism and making its hidden treasure the first geode open to visitors in the world. There are strict rules: visitors are not be able to go inside the geode, but can see its interior through a type of window, to guarantee their own safety as well as preserve this unique treasure.

"Gypsum is a very soft material, you can scratch it with your nails. Footsteps, damp and CO2 from people breathing can also damage the crystals," explained Francisco Fernández Amo, a geologist from the Tecminsa company which has carried out the project to turn the site into a museum.

Those who already know the mine are convinced it is a remarkable tourist attraction. "The mine has a large variety of minerals and during the visit people will be able to see the insides of dozens of smaller split geodes, one of which is 3.5 metres," said the local councillor for Tourism, Juan Bautista López, prior to its opening to the public this week.

The visitors have to wear safety helmets and, accompanied by a guide, walk in small groups through the 330 metres of the main gallery. They then go down to the second level, where they can take selfies with a split geode and access the 40-metre-high excavation chambers.

"They are like underground cathedrals," said Fernández Amo. "The geode is unique in the world, but the geological and mining heritage is also fantastic. There are faults, folds and different geological structures."

Not long ago, he made a further discovery: some of the minerals that form the walls of the mine are fluorescent when ultraviolet light is shone on them. Calcite, strontium calcite, epsomite and aragonite glow in hues of red, green, violet and silver which add to the spectacle of the underground route.

Inside the mine you can still see the 'cages' used to descend to the depths, machinery, boxes of explosives and tools used by the miners in different periods, as well as packets of cigarettes and old beer bottles.

Finally, visitors are led down steps to the third level - which is at sea level - to access the platform from which the 'treasure cave', the interior of the huge diode, can be seen through a glass panel.

When the dissolution of a mineral cools very slowly, the atoms are placed in an ordered manner and form polyhedral structures. There are crystals everywhere, sometimes visible such as in salt, sugar or sand, and sometimes not, as in ice cream, coral, medications and electronic equipment.

It is a hollow rock whose inside is covered with crystals, mostly quartz, calcite and gypsum. They form in different ways. In geodes like the ones in Pulpí and Naica, the water cools over thousands and thousands of years. Juan Manuel García, the geologist at the CSIC who took part in surveying the Mexican cave, said its crystals grew at the rate of "one human hair per century". That is, indeed, very slow.

metres is the distance travelled on a visit to Mina Rica, although the pit, which is also known as Quien Tal Pensara, is much bigger.

The mine, which is in the Pilar de Jaravía area of Pulpí, a few kilometres from the boundary between Almeria and Murcia, opened to visitors this week. Tickets cost between 10 and 22 euros and are currently available for dates between 23 September and 13 October. Visit www.geodapulpi.es