The mysterious island, Por-Bajin, was found in the centre of a remote lake high in the mountains of southern Siberia
At first glance the rectangular island of Por-Bajin looks like a foreboding fortress or prison, with its regular structure and ruined sections.
But it is unknown for certain what the rectangular island and its labyrinthine ruins, located in a lake in deepest Siberia, was actually used for when it was built 1,300 years ago.
Continue reading and see more images
A reconstruction sought to show what the island could have looked like before it was ruined.
Historians and scientists are divided, but some experts believe the isolated area may have been built to attract people instead imprisoning them, and suggest it was a summer palace, monastery, or an astronomical observatory.
The name Por-Bajin translates as 'clay house' in the Tuvan language, and the island sits between the Sayan and Altai ranges, about 3,800km from Moscow near the Mongolian border.
The fortress is located about 3,800km from Moscow near the Mongolian border
The location was first discovered in 1891, and the purpose of island has still not been explained over a century later.
More in-depth research took place in 2007 with archaeologists discovering clay tablets of human feet, faded coloured drawings on the plaster of the walls, giant gates and fragments of burnt wood.
Archaeological excavations in the 1950s and 60s - all inconclusive.
Experts say the island was built during the period of the Uighur Khaganate (744-840 AD) but it is not clear what their motive would be for constructing a fortress for in such a solitary place – as it is far from big settlements and trade routes.
The plot thickens as the way it was built, along with the construction materials use, hint at Chinese architectural tradition.
A prison? A palace? A monastery?
Scientists were able to use laser mapping to create a 3D image of what the 3.5 hectare plot could have been used for.
Although it is said to be around 1,300 years old, many walls were intact and well preserved with a main structure in the inner courtyard being separated into two parts, covered by a tiled roof walkway supported by 36 wooden columns on stone bases.
The fortress is connected to another island via a walkway
Even more confusing than the debate of why it was created is the question of why it was abandoned.
Researches have noted the lack of heating systems on the island, despite it enduring harsh Siberian weather and being located 7,545 feet above sea level.
Russian president Vladimir Putin visited the island with Prince Albert of Monaco in 2007 and echoed the confusion. 'I have been to many places, I have seen many things, but I have never seen anything of the kind,' he said.
The few artifacts unearthed at the site seem to date it to the mid-eighth century