They're the circle of stones that have left historians scratching their heads for centuries.

Now, experts have made a 'stunning' discovery about one of Stonehenge's most famous rocks.

It has been known for a while that most of the monument's world-renowned 'bluestones' were sourced from the Preseli Hills in Wales, around 240km (150 miles) from the site in Wiltshire.

But new analysis has revealed the Altar Stone, the largest bluestone at the centre of Stonehenge, actually came from northern Scotland – up to 1,000km (621 miles) away.

Experts have no idea why the six-tonne, 5m-long stone was selected, nor how it was transported such a long distance around 5,000 years ago.

The discovery was made when a team, led by researchers from Curtin University in Perth, analysed the age and chemistry of minerals from fragments of the Altar Stone.

They found a remarkable similarity with the Old Red Sandstone of the Orcadian Basin in northeast Scotland.

The team said they can conclude with 95 per cent accuracy that the stone came from this area – which encompasses parts of Inverness, Thurso, Orkney and parts of Shetland.

The 'fingerprint' of the mineral grains within the sandstone are completely different to Welsh-sourced stones.

'The findings are truly remarkable – they overturns what had been thought for the past century,' co-author Professor Richard Bevins, from Aberystwyth University said.

'We have succeeded in working out, if you like, the age and chemical fingerprints of perhaps one of the most famous stones in the world-renowned ancient monument.

'It's thrilling to know that our chemical analysis and dating work has finally unlocked this great mystery.

'We can now say that this iconic rock is Scottish and not Welsh.'

While the research, published in the journal Nature, does not provide direct evidence about how the Altar Stone got to its location in Salisbury Plain, the revelation it travelled so far has raised questions about its journey given the limits of human technology during Neolithic times.

Professor Nick Pearce, also from Aberystwyth University said: 'This stone has travelled an awful long way – at least 700 km - and this is the longest recorded journey for any stone used in a monument at that period.

'The distance travelled is astonishing for the time. While the purpose of our new, empirical research was not to answer the question of how it got there, there are obvious physical barriers to transporting by land, but a daunting journey if going by sea.

'There's no doubt that this Scottish source shows a high level of societal organisation in the British Isles during the period.'

Construction at Stonehenge began 5,000 years ago, with changes and additions over the next two millennia.

The researchers said the Altar Stone was most likely extracted from the Orcadian Basin using stone wedges and hammerstones to exploit natural joints within the rock.

Although it is unclear when the Altar Stone arrived at Stonehenge, it may have been placed within the world-renowned central horseshoe of stones during the second construction phase at around 2620 – 2480 BC.

Lead author Anthony Clarke from Curtin University said: 'Given its Scottish origins, the findings raise fascinating questions, considering the technological constraints of the Neolithic era, as to how such a massive stone was transported over vast distances around 2600 BC.'

Previous studies indicate the larger upstanding 'sarsen' stones, which make up the outer circle of Stonehenge, were sourced from the edge of the Marlborough Downs, around 15 miles north of the site.

The research also involved scientists from University College London (UCL) and the University of Adelaide.

The team said their next step will be further pinpointing the origin of the stone within the Orcadian Basin.

The Stonehenge monument standing today was the final stage of a four part building project that ended 3,500 years ago

Stonehenge is one of the most prominent prehistoric monuments in Britain. The Stonehenge that can be seen today is the final stage that was completed about 3,500 years ago. 

According to the monument's website, Stonehenge was built in four stages:   

First stage: The first version of Stonehenge was a large earthwork or Henge, comprising a ditch, bank and the Aubrey holes, all probably built around 3100 BC. 

The Aubrey  holes are round pits in the chalk, about one metre (3.3 feet) wide and deep, with steep sides and flat bottoms. 

They form a circle about 86.6 metres (284 feet) in diameter. 

Excavations revealed cremated human bones in some of the chalk filling, but the holes themselves were likely not made to be used as graves, but as part of a religious ceremony.

After this first stage, Stonehenge was abandoned and left untouched for more than 1,000 years. 

Second stage: The second and most dramatic stage of Stonehenge started around 2150 years BC, when about 82 bluestones from the Preseli mountains in south-west Wales were transported to the site. It's thought that the stones, some of which weigh four tonnes each, were dragged on rollers and sledges to the waters at Milford Haven, where they were loaded onto rafts.

They were carried on water along the south coast of Wales and up the rivers Avon and Frome, before being dragged overland again near Warminster and Wiltshire.

The final stage of the journey was mainly by water, down the river Wylye to Salisbury, then the Salisbury Avon to west Amesbury. 

The journey spanned nearly 240 miles, and once at the site, the stones were set up in the centre to form an incomplete double circle. 

During the same period, the original entrance was widened and a pair of Heel Stones were erected. The nearer part of the Avenue, connecting Stonehenge with the River Avon, was built aligned with the midsummer sunrise. 

Third stage: The third stage of Stonehenge, which took place about 2000 years BC, saw the arrival of the sarsen stones (a type of sandstone), which were larger than the bluestones.

They were likely brought from the Marlborough Downs (40 kilometres, or 25 miles, north of Stonehenge). 

The largest of the sarsen stones transported to Stonehenge weighs 50 tonnes, and transportation by water would not have been possible, so it's suspected that they were transported using sledges and ropes. 

Calculations have shown that it would have taken 500 men using leather ropes to pull one stone, with an extra 100 men needed to lay the rollers in front of the sledge.

These stones were arranged in an outer circle with a continuous run of lintels - horizontal supports. 

Inside the circle, five trilithons - structures consisting of two upright stones and a third across the top as a lintel - were placed in a horseshoe arrangement, which can still be seen today. 

Final stage: The fourth and final stage took place just after 1500 years BC, when the smaller bluestones were rearranged in the horseshoe and circle that can be seen today.

The original number of stones in the bluestone circle was probably around 60, but these have since been removed or broken up. Some remain as stumps below ground level. 

Source: Stonehenge.co.uk