Between 700 and 300 BC, a large number of forts were built in Scotland, many of them on top of hills, with walls made of stones piled together without the use of mortar.
This at first may not seem like anything out of the ordinary, as of course, there are plenty of such structures across the world, and not just in Europe.
Some of the areas of the forts were converted into a kind of glass, featuring the remains of what was without doubt air bubbles and drops of molten rock which are evidence that the stones were once subjected to temperatures that led to a vitrification process.
Therefore, during the last three centuries, archaeologists have tried to answer the questions surrounding the mysterious Scottish fortresses.
One of the first British geologists to describe these mysterious structures and the mystery behind them was John Williams, author of Natural History of the Mineral Kingdom.
It was he who first described the mystery in 1777 after looking at a few strange ruins, of which later more than a hundred examples have been found throughout Europe, mainly in Scotland.
So, who built them? How did they manage to vitrify the stone? And what sort of technology was used? And is it possible that we do not see the entire picture?
Too many questions and no answers at all.
These structures were given the name of vitrified forts. These structures have amazed geologists for centuries because there is no scientific explanation for how the rocks fused.
The temperatures at which they had to be submitted for vitrification occur are comparable to the detonation of an atomic bomb, say some experts.
But what is interesting is the fact that there aren’t one or two vitrified structures, but hundreds of examples spread across Europe, with 70 forts existing in Scotland.
As the first vitrified structures were discovered in Scotland, it was thought that they were exclusive to Scotland, the most famous being Dun Mac Sniachan, Benderloch, Craig Phadraig, Ord Hill, Dun Deardail, Knock Farril, Dun Creich, Finavon, Barryhill, Laws, Dun Gall, Anwoth, Tap or O’Nort.
However, examples of similar structures have been found in Bohemia, Silesia, Thuringia, in the provinces of the Rhine, in Hungary, Turkey, Iran, Portugal, France, and Sweden, among others.
Experts have found that in some cases the stones appear partially calcined and fused, while in others they are covered by a layer of vitreous enamel, and sometimes, although rarely, the entire length of the wall presents a solid mass of a vitreous substance.
Nobody knows how these walls came to be vitrified.
Some scholars believe that it was intentional, to strengthen the defenses of the forts, but in reality, this would have weakened them, so it is unlikely that this was their intention.
Experts also say that the vitrification was unlikely to be the result of war damage, as the result of a siege, because in order to reach vitrification, the fires must have remained burning for days at a temperature between 1050 and 1235 degrees Celsius, something that is extremely improbable, although not impossible.
Some theories point to the possibility that the vitrification of the first may have been the product of deliberate destruction either by attackers after the capture of the forts or by their occupants as a ritual act.
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The dating of the forts across Europe covers a wide range of dates.
The oldest forts are believed to have been built during the Iron Age, but there are also those many forts with similar characteristics dating from the Roman era, while the last corresponds to the Middle Ages.
Recent studies suggest that they were created by massive plasma events such as solar flares.
These occur when the ionized gas in the atmosphere takes the form of gigantic electric bursts, which can melt and vitrify rocks.
In the 1930s archaeologists, Vere Gordon Childe and Wallace Thorneycroft conducted an experiment with a gigantic fire directed towards a stone wall, an experiment that was repeated in 1980 by the archaeologist Ian Ralston.
In both cases, the experiments produced the partial vitrification of some of the stones, but they failed to explain how it could have been produced on such a large scale as in the vitrified forts.
In the absence of a definitive theory or conclusive evidence, the vitrified forts of Europe continue to be one of the strangest geological and archaeological anomalies in the world, eluding explanation for centuries.