A mystery has puzzled historians since the 18^th century, when a survey in the Scottish Highlands found an ancient fort with walls made from stones that had been melted together. Now scientists believe they have found the explanation for this odd formation.

The discovery of the fort was at first dismissed as pure fiction; others speculated that it was actually a natural phenomenon, the remains of an extinguished volcano.

But many more such forts were found, eventually numbering around 200 in Europe, with 70 in Scotland.

How did Iron Age people, who built these structures in the first millennium B.C., manage to heat entire stone walls to 2,000 degrees Fahrenheit (1,100 degrees Celsius)?

And why did they do it, when melting the