It's the monument that has baffled scientists for hundreds of years. 

But now, experts may have finally solved one of Stonehenge's greatest mysteries.

Researchers have revisited the debate over the origin of the landmark's iconic bluestones – the group of smaller stones that are dotted around the site on Salisbury Plain in Wiltshire.

They focused on a rock known as the Newall boulder, which was uncovered during excavations around a century ago.

One of the main debates has been whether this rock – and other similar stones – reached the site after being transported by glaciers from Wales, or whether humans moved them more than 5,000 years ago.

A team, led by Professor Richard Bevins from Aberystwyth University, compared the Newall boulder – which is about the size of a football – with samples from a rocky outcrop in Wales.

Through geochemical and microscopic analysis, they concluded 'there is no evidence to support the interpretation that it is a glacial erratic'.

Instead, the stone is a precise match for the unique characteristics of rocks from Craig Rhos–y–Felin – indicating humans transported the heavy boulder from more than 125 miles (200km) away.

There is already evidence of stone quarrying at the site, in north Pembrokeshire, by Neolithic communities around 3000 BCE, they explained.

Chemical analysis revealed that the Newall boulder and other fragments found at Stonehenge share identical levels of thorium and zirconium with the stone at Craig Rhos–y–Felin.

The researchers said the Newall boulder is a foliated rhyolite and has a surface layer rich in calcium carbonate – the result of its long burial in the chalky soils at Stonehenge.

This further reinforces the idea that it was brought here by humans and not glaciers, they said, because glaciers would have left a much more dispersed distribution of similar stones across the region – something that has not been found.

The study, published in the Journal of Archaeological Science: Reports, also makes an important discovery about another of the buried stones at Stonehenge – Stone 32d.

It was previously thought to be a type of rock called a spotted dolerite, but new analysis confirms it is also a foliated rhyolite – like the Newall boulder.

Transporting these stones, which can weigh over three tonnes, from Wales to Stonehenge would have been an incredible feat of human logistics.

'Part of the fascination of Stonehenge is that many of its megaliths, in contrast to the large, relatively local sarsens, can be proven to have been sourced from Wales, over 200km to the west,' the researchers wrote.

'Most archaeologists accept that Neolithic people achieved this remarkable feat, transporting blocks weighing up to 3.5 tonnes.

'There is, however, a contrary view, which proposed that no human effort was involved.

'Rather, it is argued, the Welsh stones were transported by glacial ice long before Stonehenge was built.'

One of the main proponents of the glacier theory is geologist Dr Brian John, who has previously insisted that the Newall boulder shows glacial abrasion marks.

A prior study of his concludes: 'The boulder was reduced in size and heavily modified during glacial transport, for much of the time on the bed of a glacier.

'It was eventually dumped at some location on, or relatively close to, Salisbury Plain.'

As part of the new study, the researchers said these features could also be the result of natural weathering.

They also said that even if the rock had been transported by ice in the past, it would not explain why similar fragments are only found at Stonehenge and not anywhere else on the Salisbury Plain.

Their findings indicate that Dr John's arguments 'have no basis in evidence', they wrote, adding: 'To present it as fact, rather than as hypothesis, is disingenuous'.

'The presence of Stonehenge itself is the evidence of movement by Neolithic peoples of stones weighing up to as much as 40 tonnes,' they wrote.

'Unless it is argued that all the stones were just lying on the ground ready to be erected where they were to make Stonehenge, the stones must have been moved into position.

'If Neolithic people could move a stone a few tens of metres they could move it tens or hundreds of kilometres.

'It may not have been easy, but it was entirely possible and moving 2–3 tonne bluestones would involve an order of magnitude less effort than moving the sarsens.'

While the large sarsen stones were only transported from West Woods in Wiltshire – roughly 32km (20 miles) away – they weighed over 20 tonnes each and stood up to seven metres tall.

The famous 'Altar Stone' was even transported from Scotland around 750km (466 miles) away, a study published last year revealed. 

'There is no evidence for how they moved these stones,' the researchers wrote, 'but recent indigenous peoples have been transporting stones weighing many tonnes great distances with ropes, wooden sledges and trackways – technologies which would have been available in the Neolithic.'

While builders 5,000 years ago normally used whatever materials were at hand for stone circles, Stonehenge is 'exceptional' for being constructed entirely of stones brought from long distances, the team said.

Researchers have previously suggested that the site may have had a political as well as a religious purpose, 'a moment of unification for the peoples of Britain, celebrating their eternal links with their ancestors and the cosmos'.

The new study provides strong evidence of human transport, but the mystery of exactly why Stonehenge was built – and how – is ongoing.

The team concluded: 'We reiterate our previous interpretation that the Newall boulder is not a glacial erratic, that there is no evidence of glaciation on the Salisbury Plain, and that the bluestones were transported to Stonehenge by humans and not by ice.' 

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