Archaeologists have begun a historic dig which they hope will unravel the ancient mystery of Stonehenge, on Salisbury Plain.
A trench is being excavated at the World Heritage site in a bid to establish the precise dating of the Double Bluestone Circle, the first stone structure that was built there thousands of years ago.
Timothy Darvill and Geoffrey Wainwright, the professors leading the first dig inside the stone circle in 44 years, believe their project could answer the "eternal questions" of when and why Stonehenge was first built.
Prof Darvill, of Bournemouth University, and Prof Wainwright, President of the Society of Antiquaries, will compare samples with their research in the Preseli hills in south-west Wales, from where 80 such stones were carried an estimated 4,500 years ago.
Prof Wainwright said: "The excavation will date the arrival of the bluestones following their 250km (153 mile) journey from Preseli to Salisbury Plain and contribute to our definition of the society which undertook such an ambitious project.
"We will be able to say not only why but when the first stone monument was built."
The last time an excavation was allowed inside the sarsen stone pillars was in 1964.
The hole, which will eventually measure 3.5 metres wide and 1.5 metres deep, will be dug by hand in a previously excavated area on the south-eastern quadrant of the Double Stone Circle with the hope of retrieving fragments of the original bluestone pillars.
English Heritage agreed to the excavation on Salisbury Plain, Wiltshire, following consent by the Secretary of State for Culture, Media and Sport.
Stonehenge will remain open as normal and visitors will be able to observe up close the excavation as it happens on plasma screens inside a special marquee.
BBC Timewatch and Smithsonian Networks will fund the project.