Stonehenge mystery deepens: Scientists say the altar stone did NOT come from Orkney as claimed - as the hunt for its place of origin continues

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