Glyn Goodrick, an archaeologist at Newcastle University, was working alongside his archeological team's mechanical digger when it scraped the first sarsen stone to emerge from the topsoil covering the site, which lies about half a mile south-west of Avebury.
"The sound of the digger's bucket moving the earth changed suddenly, so I could tell straight away there was something there. The driver and I just looked at each other, stopped the machine and dived straight in with our shovels, scraping away the surrounding soil to reveal the top of the stone," Mr Goodrick said.
"It's the most exciting thing I've seen in 16 years working in archaeology. Taken with what we have found since, this has to be one of the most significant finds of a prehistoric site this century in England." Because of the find, the complex of prehistoric sites dating back 6,000 years that surrounds the stone circle at Avebury - including Silbury Hill, Windmill Hill, the West Kennet long barrow and the Sanctuary - will have to be reinterpreted.
One week on, a total of six sarsen stones - buried in pits intact or in the form of burnt remains - have been revealed by the 15-strong archaeological team of academics and students organised by the universities of Leicester, South-ampton and Wales.
The discovery establishes the existence of "Beckhampton Avenue", suggested and named by the 18th-century Lincoln-shire antiquarian, William Stukeley. The location and spacing of the stones unearthed this week are exactly in line with the theoretical avenue linking the Beckhampton long barrow with the Avebury henge mooted by Stukeley in the 1720s. They also form a precise alignment between the centre of the henge to the east of the current excavation and two large megaliths - "Adam" and "Eve", together known as the Longstones - slightly to the west.
From the medieval period onwards, many of the sarsen stones erected in and around Avebury were broken up to use in construction, or burnt and buried in pits. Both during his lifetime and beyond, Stukeley's ideas were regularly pilloried by fellow historians and antiquarians as wishful thinking. This discovery of a second avenue of parallel pairs of sarsen stones, to the south-west of the henge, will reinforce speculation of a possible third to the north of the village, and will enhance Avebury's status as a World Heritage Site. The second avenue cuts across an oval Neo-lithic enclosure, 150 metres long and 100 metres wide, within the field. Shards of Neolithic pottery have already been unearthed.
Joshua Pollard, of the University of Wales, Newport, who is a co-director of the excavation, said: "I think part of the purpose of these avenues must have been to link together existing monuments, sacred places that made up a unified ceremonial complex. The Beckhampton Avenue is a very significant discovery because it forces us to rethink the whole nature of the Avebury complex. It means the scale of monumental construction during the later Neolithic period [3000BC to 2200BC] was much greater than was previously thought.
"We may be able to understand better the symbolic meaning the monuments carried for the people who erected and visited them, and the way that the structures regulated movement and experience of life. We would never have been led to its discovery had it not been for the records which Stukeley kept," Mr Pollard said.
More than 250,000 people visit Avebury each year. John Aubrey, another 17th-century antiquarian, described Avebury and its surrounding archaeological gems as a "cathedral", compared with the "parish church" of Stonehenge.
Growing interest in the National Trust-owned henge at Avebury has prompted concerns about the security of the monuments themselves, and about erosion of the earthworks in which they stand. Vandals recently covered one of the biggest circle stones with paint.