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(Courtesy El Soplao Cave)

This is hardly a mountain we're standing on up here in northern Spain. Compared to the snow-capped Picos de Europa visible to the southwest, which tower over 2,600 metres, this green mound in the Sierra de Arnero is barely a geological speed bump.

In fact, at a mere 540 metres, this hill is as tiny as Toronto's CN Tower and as such, doesn't even warrant a name in this lofty landscape.

Still, it's worth taking a trip up this no-name mountain, which rises above the village of Rábago, some 60 kilometres southwest of Santander, the capital of Cantabria. The views are spectacular, but the really remarkable attraction is hidden underfoot.

Cantabria is a land of caves. Lying midway along the Iberian peninsula's northern coast, the region — one of Spain's 17 provincial comunidades autónoma (autonomous communities) — measures just 5,000 square kilometres, yet it holds more than 6,500 caves.

The most famous is Altamira, with its astonishingly preserved paintings  of bison and horses from the Paleolithic era. But since Altamira was shuttered to tourists a few years back and replaced with a facsimile, to protect the original from deterioration, another cave called El Soplao has been muscling its way into the speleological limelight.

What's inside

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Speleothems are mineral formations that can grow into a variety of shapes. (Courtesy El Soplao Cave)

El Soplao, which is inside no-name mountain, is more than just a cave.

"Soplao" is Spanish for the draft created when a mine shaft opens into a natural underground cavity. From 1857 to 1978 this site was known as La Florida mine, where men burrowed some seven kilometres, taking out zinc and lead. When in 1908 their tunneling broke into a massive and mysterious cavern, the miners didn't know it but they had just made one of the rarest geological discoveries in history.

That's because, according to cave experts — speleologists — El Soplao contains the most extensive collection ever found of eccentric crystallographic formations, called helictites.

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Helictites or eccentrics are speleothems that grow in wildly erratic directions. (courtesy El Soplao Cave)

"Eccentric formations are quite uncommon in caves around the world," says Sergio Garcia-Dils de la Vega, an archeologist with the University of Seville, and one of Spain's leading speleologists. "So, the huge number of helictites is what makes El Soplao very special."

Most people are familiar with stalactites and stalagmites, those vertical spikes formed when mineral-laden water drips from the roof to the floor of a cave. Called speleothems, such formations can also grow into other shapes, such as rippling curtain-like sheets (called drapery), or spherical "cave pearls" (pisolites), or "frozen waterfalls" of minerals (flowstone).

While El Soplao cave, which measures some 1,500 metres long by 30 metres high, contains thousands of such formations, what makes it truly unique is its millions of helictites.

Eccentrics galore

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'Soplao' is Spanish for the draft created when a mine shaft opens into a natural underground cavity. (Courtesy El Soplao Cave)

Helictites — often just called "eccentrics" — are speleothems that defy gravity. They bend off the vertical axis and grow in wildly erratic directions - sideways, curves, zigzaggs, and in starbursts, spirals, hooks and loops.  No one knows why helictites grow in such an unusual manner.

"Some scientists suggest it is due to drafts of air inside the cave," says Garcia-Dils de la Vega. "Others say the explanation lies in the presence of electric and magnetic fields."

It is also not known exactly how old the El Soplao formations are. The cave itself was born with the mountain around 100 million years ago, but since speleothems grow at unpredictable rates, scientists can only peg its formations at between one million and 50,000-years old.

"The average density of eccentrics in El Soplao is about 4,000 formations per square metre," says Garcia-Dils de la Vega, who has himself explored deep into the cave and mine. "It is one of the greatest measured in the world."

Due to their high calcium carbonate content, the El Soplao helictites are pure white. Much smaller than other speleothems, they bristle off the walls and ceiling of the cave like thick frost.

Geological research centre

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The cave has a train to bring visitors to its heart, and a series of walkways through the formations. (Courtesy El Soplao Cave)

For those miners in 1908, one imagines it must have been an ominous discovery, viewing this crepuscular netherworld only by the dim beams of their oil lamps. Even today, although manmade paths have been laid in the cave and discreet electric lighting installed, you still can't help imagining all manner of incongruous and phantasmagorical objects as you gaze into the strange mixture of formations that surround you — spider legs, pom-poms, melting wax, giant squids, Celtic knots, dove wings, parsnips, ghosts. The miners surely knew they'd found something extraordinary, for they left the cave almost completely untouched.

These days, El Soplao is a thriving centre for geological research. It opened in 2003 with a team of 60 scientists, and it is a museum visited year-round by some 300,000 tourists. Some people tour just the cave itself, transported down to its opening through a long tunnel via a small locomotive, while the more adventurous don hard hats and rubber boots to trek by foot deep into the mine.

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The average density of eccentrics in El Soplao is about 4,000 formations per square metre. (Courtesy El Soplao Cave)

Despite this influx of humanity, El Soplao's pastoral surroundings remain remarkably untouched, the lush, green Nansa Valley below dotted with dairy farms and quaint villages. Even outside the cave, amazing discoveries are being made. As our tour bus winds back down no-name mountain into the valley, it stops briefly and the guide points to a blue tarp covering a patch of the grassy slope.

"In 2008, paleontologists here discovered the world's oldest specimens of insects trapped in amber," he says, "extinct 110 million years."

Indeed this may be a small mountain, but considering its enormous scientific significance, someone really ought to give this speed bump a name.