Deep below ground in a coal mine in Siberia, excavators have uncovered something remarkable: ten giant spheres of stone, each incredibly smooth and a metre across. Guessing what these mysterious rocks might be has brought out the internet's creative side:


Workers moved the spheres to the surface where passers-by can marvel at them, reports The Siberian Times, and it turns out the rocks take on a rusty-red colour when it rains. But eggs of prehistoric beasts or buried artefacts of an alien civilisation these are not – and any geologist will tell you they're not only perfectly natural, but also pretty common.

Known as concretions, the huge orbs are formed deep underground over millions of years, often when sediment builds up around a centre. Think of a small stone picking up snow as it rolls down a hill, forming an ever-larger snowball. But in the case of a concretion, the central stone stays in place deep below, while flowing groundwater brings in sediment from all directions, slowly fusing them together with mineral cement.

Most concretions are very small, but given plenty of time and an ample supply of mineral-rich groundwater, they can grow to enormous sizes. And because they're made of tightly cemented sediment, concretions often hold up strongly while the rock around them erodes and crumbles.

You'll find these cool formations all over the world, and the geologic processes that form them can generate pretty intriguing shapes and patterns. Some are so famous they've earned their own names.

The Navajo Sandstone of the western United States is well-known for its reddish "Moqui marbles", tiny concretions of the rusty mineral hematite, formed in Jurassic lake sediments. In South Africa, very similar concretions called "Klerksdorp spheres" are so unusual-looking that some people insist they were left by aliens. In fact, they were formed in volcanic sediments three billion years ago.

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Moqui Marbles from the Navajo Sandstone of Utah. Image: Luc De Leeuw, Flickr

But concretions of enormous size are much more rare. On the coast of North California, the crashing waves of the Pacific erode away the cliffs, slowly exhuming dozens of round concretions each over a meter across, giving rise to the name "Bowling Ball Beach". And in New Zealand, a similar process is exposing the "Moeraki Boulders", huge concretions covered in gorgeous mud-crack patterns, some of them over two meters in diameter.

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Moeraki Boulders on the coast of Otago in New Zealand. Image: Karsten Sperling, Wikipedia Commons
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The concretions of “Bowling Ball Beach” in California. Image: Brocken Inaglory, Wikipedia Commons.

The Siberian "pearls", which were found 30 metres (98ft) underground, are huge. Like the Moeraki Boulders, they spent perhaps millions of years growing to their final size. And like the Moqui Marbles, they are partially made of iron oxide, which is what causes them to "rust" in the rain.

To a rockhound, an interesting concretion may be an exciting collector's item, but to a fossil hunter, it can be a frustratingly bone-shaped distraction (I speak from experience!). And for those with active imaginations, large concretions like those in Siberia are alien pods or dinosaur eggs.

But there’s no need for fantasy. The planet's natural processes are incredible enough to create plenty of real-life wonders of the world.  


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Top header image: The Moeraki Boulders at Moeraki Beach, New Zealand. Nuytsia@Tas, Flickr