For more than 800 years, prehistoric people in southern Britain had used the exposed site on Salisbury Plain, Wiltshire - today known as Stonehenge - as a place for their rituals. Over those eight centuries two circular banks of earth had been built and two incomplete circles of stones had been erected.
But in about 2000 BC, the most challenging task was still ahead. It was then that workers began the job of erecting the largest structures of Stonehenge - the five trilithons forming a horseshoe at the centre of the circle. Each consists of two 50 ton upright stones around 20ft (6m) high with a 7 ton stone resting across the top.
Raising a 7 ton lintel some 20ft (6m) onto its pair of uprights was probably the most dangerous and demanding job in building Stonehenge. Most likely, each lintel was raised on a bed of logs, each end of the lintel being levered up alternately while logs were pushed under it.
The three phases of Stonehenge
Stonehenge was built in three distinct phases over a period of about 1700 years. Professor Gerald Hawkins, formerly of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, USA, has estimated that the entire monument would have taken a total of about 1,500,000 working days to construct, and involved about 1000 workers at a time.
Phase I was begun about 2750 BC, nearly 200 years before the Egyptians started work on the Great Pyramid. It is a circle some 380ft (115m) across consisting of a low outer bank surrounding a ditch, with another bank about 6ft high inside the ditch. Inside the inner bank, the Stone Age Britons dug 56 equally spaced pits, called the Aubrey Holes after the 17th-century antiquary John Aubrey, who first noticed them as slight dips in the turf. What they were for is not known.
There is an entrance on the north-east side of the circle. Outside it stands a huge block of rough sandstone about 16ft high, known today as the Heel Stone. From the centre of the circle, you can see the sun rise over it on Midsummer's Day.
Phase II of the construction began around 2100 BC, and was carried out by the Beaker Folk, who were so called because of the shape of their pottery. They erected 80 large bluish stones - known today as the bluestones - in two incomplete rings in the centre of the monument. They also built a wide roadway, now called the Avenue, leading north-east towards the River Avon about 2 miles (3 km) away.
The bluestones came from the Preseli Mountains in south-west Wales, 130 miles distant, and were probably brought most of the way by water - loaded on rafts at Milford Haven and shipped up the estuary of the River Severn. By using a network of rivers, only a short overland journey was left, from Amesbury to Stonehenge along the Avenue.
Support for this theory was provided in 1988 when a bluestone block was discovered on the bed of the River Daugleddau at Llangwm in Dyfed. It is of similar size to those used at Stonehenge, and its position suggests that it could have sunk while being floated down the river to the sea.
The pale green, broken Altar Stone, once standing but now lying flat among the central trilithons, came from the shores of Milford Haven, probably also by water.
Phase III, lasting from about 2000 to 1100 BC, was carried out by early Bronze Age people. They removed the bluestone circle and erected a ring of about 30 sandstone uprights (averaging 30 tons in weight), linked by stone lintels. The ring is 16ft high overall, and inside it they put up the five even taller trilithons. Finally they re-erected the bluestones in two groups.
The sandstones, or sarsens, came from the Marlborough Downs, 20 miles (32 kms) away. They must have been manhandled on sledges, with oak logs used as rollers. Professor Hawkins has calculated that 800 men would have been needed to haul one of the giant 50 ton sarsens. Another 200 would have had to clear the route and continually move the heavy oak rollers from the back of the stone to the front.
The stones were shaped by chipping away at the surface with other stones. Larger lumps may have been split off by heating the stones along carefully marked lines, throwing cold water on them and then hitting them.
Christopher Chippindale's Stonehenge Complete gives the derivation of Stonehenge as coming from the Old English words "stān" meaning "stone", and either "hencg" meaning "hinge" (because the stone lintels hinge on the upright stones) or "hen(c)en" meaning "gallows" or "instrument of torture".