Dun Deardail, a hilltop fort in Glen Nevis, is dated to 500 BC and likely served as a Celtic fort and then a Pictish citadel.
It continues to defy explanation given it was made from stone and timber burnt at such intense heat that the rocks melted and fused together in a process known as vitrification.
Theories on how its inhabitants were able to create such temperatures - around 1,000 degrees Celsius - over long periods have divided opinion among archaeologists and geologists for decades with conspiracy theorists too interested in the answer.
This month, a team from AOC Archaeology, Nevis Landscape Partnership and Forestry Commission Scotland will return to Dun Deardail for the third time to carry out excavation work at the site.
A spokeswoman for Nevis Landscape Partnership said: “Vitrified forts, like Dun Deardail in Glen Nevis, continue to bewilder even the sharpest mind but this month Nevis Landscape Partnership & Forestry Commission Scotland will attempt to understand this fascinating process and the people who inhabited this impressive Iron Age settlement when we begin our third and final year of excavations.”
Dun Deardail, which dates to 500BC, sits on the West Highland Way on an elevated rocky knoll to the west side of Glen Nevis.
The hillfort is overlooked by Ben Nevis and has breath-taking views over the surrounding glen.
It is around one of 60 vitrified forts in Scotland - with others including Tap O’ North in Aberdeenshire and Ord Hill near Inverness. Similar structures are found in parts of France, Germany Wales and Ireland.
Remains of other vitrified forts can be found along the Great Glen Way with Dun Deardail providing a defensive site - and probably a status symbol - for its elite occupants.
Andy Heald of AOC Archaeology said several explanations have been given as to why the rock was burnt at such places.
Mr Heald said: “Some people think vitrification was a status symbol, some people think a settlement would be set alight and inadvertently vitrified in the process by attackers and some people think it’s a structural thing to do with strengthening the walls of the fort.”
With no clear answer, conspiracy theorists have their own views.
In addition, science fiction writer and inventor Arthur C Clark took an interest in Scottish vitrified forts in the early the 1980s and recreated and earlier experiment with archaeologist Dr Ian Ralston at Tap O’ North.
A rampart wall of stone and timber beams was built with around 6 tonnes of wood burnt on top of it for more than 22 hours.
Some vitrification took place but the test could not explain how the process could work on the scale of a fort - or how such large amounts of fuel could be transported to a hilltop during prehistoric times.
Clark, when asked in 2004 what he considered to be his biggest unsolved mystery, said: “The oddest thing is these vitrified forts in Scotland. I just thought, how? After all, lasers were not common in the Stone Age.”
Vitrified rock has also been found at the Incan fort of Sacsayhuaman in Peru. Conspiracy theorists have suggested the feat of engineering could have been achieved using a vast network of mirrors and lenses to concentrate sunlight - or that an alien invasion could have been responsible.
The return of archaeologists to Dun Deardail will be marked with at an Archaeology Festival at Glen Nevis on August 19.