The reason why Stonehenge was built over 5,000 years ago remains one of archaeology's most fascinating mysteries.
Now, one scientist has proposed a very surprising new theory.
Professor Terence Meaden, an archaeologist and retired physicist from the University of Oxford, claims that Stonehenge may have been a phallic temple.
Professor Meaden claims the central stone would have resembled a 2.6-metre-long (8.5ft) anatomically correct penis, complete with an 80-centimetre-long glans or bulbous tip.
According to his analysis, ancient Britons would have used tools to remove about 200,000 cubic centimetres of material to give the pillar its suggestive shape.
Although the monolith known as Stone 67 is currently lying half-buried on its side, it would have once stood at the most important position in the structure.
Before toppling over, Stone 67 would have aligned with the sunrise at the summer solstice and the sunset at the winter solstice.
If this is correct, it would suggest that Stonehenge could be a vast fertility temple, designed to highlight auspicious days in the Stone Age calendar.
Scientists say that Stonehenge might have been an ancient fertility temple after finding that a key stone was carved to look like a giant penis
Professor Terence Meaden, an archaeologist and retired physicist from the University of Oxford, says that Stone 67 (pictured) would have resembled a 2.6-metre-long (8.5ft) anatomically correct phallus, complete with an 80-centimetre-long glans or bulbous tip
The reason why Stonehenge was built over 5,000 years ago remains one of archaeology's most fascinating mysteries
While it might not be obvious now, Professor Meaden believes that the inconspicuous Stone 67 was once a critical part of Stonehenge's ritual significance.
Based on its 'contoured shape' and the presence of 'peck marks' - indentations left by the impact of stone tools - he argues that the stone must have been deliberately shaped.
Speaking to MailOnline, Professor Meaden even argues that the shaft contains 'realistic impressions of skin.'
However, due to human or natural causes, two nearby stones weighing around 20 tonnes seem to have fallen onto the stone, knocking it to the ground.
Professor Meaden says: 'When knocked down, Stone 67 fell between other stones, making the detailed features difficult to see.
'Because of this, and perhaps Christian prudery, it was never previously noted by the millions of visitors over the last five centuries.'
Archaeologists haven't found any other phallic standing stones on the Salisbury Plain where Stonehenge is located.
However, several smaller carved penises from around the time of Stonehenge's construction have been found just two miles from the site.
Professor Meaden says that the stone penis has been carved to have 'realistic impressions of skin' and bears signs of working with stone tools
Stone 67 would have sat on the central solstice axis (solid red line) which aligns with the rising sun on the Summer Solstice and the setting sun on the Winter Solstice
Although no phallic monuments have been found near stone henge, depictions of penises were common prehistoric sites throughout Europe and Asia, such as this sculpture from Gobekli Tepe, Turkey
What is the Altar Stone?
The Altar Stone is a six-tonne sandstone slab that lies flat at the centre of Stonehenge.
The five-metre-long rectangular Altar Stone is a grey-green sandstone, far bigger and different in its composition from the other bluestones.
Researchers say the Altar Stone came from Scotland - potentially transported south by a Scottish tribe as their contribution to building Stonehenge.
The Altar Stone may have once stood vertically at Stonehenge before falling to lie parallel with the ground.
Likewise, phallic monuments are relatively common in other areas and seem to have played an important role in the religious belief systems of Neolithic Europe and Asia.
Ever since serious scientific study of Stonehenge began, archaeologists have noted its alignment with the movements of the sun.
These movements are believed to have played a critical role in the Stone Age calendar since they marked the transitions between the seasons.
During the summer solstice, the longest day of the year, the sun rises behind the 'Heel Stone' in the northeastern part of the horizon, framing the rising sun as it shines its first rays into the heart of the structure.
According to Professor Meaden, the phallic Stone 67 would have been one of the few stones that actually stood directly on the solstice axis.
During the winter solstice, the setting sun would have been spectacularly silhouetted against the light and blocked the view for visitors approaching on the solar axis.
The only other stone which sits on this axis is the so-called 'altar stone', which lies on its side near the centre of the monument and also appears to have been deliberately shaped.
Recent analysis shows that this stone had been transported all the way from the Orcadian Basin in Scotland to the Salisbury Plain.
During the winter solstice, the setting sun would have backlit the standing stone and provided a dramatic site for anyone approaching on the sacred axis towards the sun
The only other stone on this axis is the 'altar stone' (circled in red) which lies on its side near the centre. Professor Meaden says that this would have been the 'female' partner' to the standing phallus stone
The enormous effort this would have taken suggests that the altar stone likely had a vital ritual importance.
According to Professor Meaden, the horizontal altar stone was the 'female partner' to the vertical phallic standing stone.
In some ancient European and Asian cultures the sun and moon often represent male and female forces respectively and there are frequent pairings between a male solar deity and a female lunar deity.
And while Stonehenge is known to align with the sun, recent studies also indicate an alignment with the movements of the moon.
In particular, the rectangular inner structure marked by the four 'station stones' could align with an event called the lunar standstill which occurs once every 18 years.
This suggests Stonehenge was a fertility temple built to reflect a symbolic connection between the female moon and the male sun at certain important times of year.
Professor Meaden says: 'All such temples have an inner sanctum where one or more icons to deities are held.
'In essence, the Sun was seen as a divinity returning every morning with radiant light and warmth which we all need. What is the truly basic need of all peoples - above all farmers? It is the fertility that leads to prosperity.'
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.
Stonehenge (pictured) is one of the most prominent prehistoric monuments in Britain
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