The ancient site at Avebury in the English county of Wiltshire is recognised as the largest surviving megalithic complex in the world. Its massive outer bank and inner ditch, which would have measured 6-8m (20-25ft) high and 9m (30ft) deep, remain a striking feature on the landscape even though they have been eroded to one third of their original size.

The scale of the site is truly remarkable. With a radius in excess of 200m (650ft) it could contain a site the size of Stonehenge several times over, and apparently has done in the past as two rings of stones to the north and south of radius 49m (161ft) and 54m (177ft) respectively have been discovered (though very few of the stones remain). Another circle, this time of two concentric rings of timber, has been revealed through geophysical surveys to the northeast of the site. Most of the original outer circle of natural Sarsen boulders set just within the ditch perimeter has been removed over the years to provide building material, but around 30 remain, the likely locations of others marked by concrete pillars.

The original site lay abandoned for thousands of years until the arrival of the Saxons who were no doubt instrumental in the destruction of the stone rings as a source of building material. Even more stones were removed to allow ploughing and cultivation of the area during the 18th century (although it is interesting to note how there was less desire to reclaim land by dismantling the bank and filling the ditch!). The site today is criss-crossed by the village High Street and Beckhampton Road reflecting its original use as a very public place.

Although the ditch and bank present an almost impenetrable obstacle to invaders the site would be better fortified had the bank been placed innermost offering defenders an even greater height advantage over their enemies in the ditch but what else could have possessed the original inhabitants to excavate around 200,000 m³ (7 million ft³) of chalk? The answer might be a problem threatening the English landscape today - floods.

Avebury is located on the edge of the water-meadows of the Kennet valley within 300 meters (1000 feet) of the River Kennet, which feeds into the River Thames and on to the North Sea. The threat posed by an elevated water table can be seen on a survey map where the area defined by the contour line 5m (16ft) above the current level of the River Kennet has been shaded. The water borders the western section of the bank suggesting it might have been constructed to prevent flooding. However, the inner area of the site is nowhere more than 6m (20ft) above the current level of the river, which means that at a depth of 9m the original ditch would have been 3m below that of the river and, more likely than not, below the level of the water table as well. That implies the ditch was, at least, intended to hold water at a depth equal to that of the adjacent river.

The Avebury megalithic complex has been placed along with neighboring earthworks and stone and timber structures in the late Neolithic period around 5,000 years ago. Analysis of the growth patterns in oak trees growing on peat bogs in Northern Ireland[2] is indicative of adverse weather conditions at 1153BC, 1628BC, 3199BC, and 4377BC. These dates and others have been associated with acidity peaks in ice cores, frost rings in ancient California bristlecone pines, and with the eruptions of Hekla in Iceland and Santorini in the Aegean Sea. The Smith Creek eruptive period (2600-1600BC) of Mount St. Helens, Washington, US witnessed the largest eruption of this volcano in history. The peat bogs have also yielded microscopic fragments of volcanic glass shards from a further eruption of Hekla. This, together with evidence from an oak tree damaged around 2350BC, could account for a dramatic downturn in the growth of the oaks lasting over a decade. These events are characterized by long periods of rain which would have seen the English countryside flooded to a far greater extent than occurs at present.

Though floods anywhere in the developed world are, today, considered a disaster, washing out roads, disrupting services and laying waste to large urban areas, they may well have been exploited by less organized civilizations in ancient times. Water was the preferred method of transport inland from estuaries for thousands of years before the development of the railway and artificial canals for diverting water have been discovered in Mesopotamia circa 4000BC. Britain during the Industrial Revolution became criss-crossed with canals including the Kennett & Avon that links rivers of the same name. At a width of 20m (70 feet) the ditch at Avebury suggests a purpose more in keeping with a canal. So, rather than protecting the site from floodwaters, the bank was the result of excavating a ditch constructed and deliberately flooded to create a terminus for boats. It would explain why the inner edge of the bank (and possibly the opposite side of the ditch) was reinforced by a rough wall of chalk blocks[3] to prevent damage from passing boats and barges. The large stones placed around the perimeter of the site just inside the ditch would have been used to moor boats, and might have been topped with small fires to illuminate the perimeter at night, as might the stone circles within the site where people would gather to trade goods. The largest stones placed at some of the present entrances to the site marked gaps in the bank where it would have been possible to enter on foot across a ford. The remains of two concentric circles of timber posts, determined during a geophysical survey of the area (but yet to be confirmed by excavation), could be the site of a large timber hall built on stilts to avoid damage by rising water levels. Other inexplicable structures within the area may also have served a purpose in this water-logged landscape. Two massive rings, identified during aerial photography of West Kennett and later the subject of investigation, straddle the River Kennet and are believed to have held timber posts up to 1m (3ft) wide and 7.5m (25ft) high. That would place their tops above the level of the surrounding water perhaps to create a quay or wharf. Silbury Hill 1.5km (0.9 miles) due south of Avebury is one of the tallest and largest man-made mounds in the world. At a height of 40m (130ft) the top would have presented a visible marker for people navigating towards Avebury from several kilometers (less miles!) away and even more so when illuminated by a fire - the worlds first lighthouse.

So, you might ask, what happened to all the boats[4]? Unfortunately wood does not stand the test of time, however, the last of three boats to be found by amateur archeologist Ted Wright in the muddy shores of Ferriby, UK in 1963 turned out to be 4,000 years old[5]. It was constructed from thick Oak planks sewn together with twisted Yew branches and at a length of 15m (50ft) and with a beam of more than 2m (6½ft) could have carried 18 men with a substantial payload. The techniques used in its construction were more reminiscent of early Egyptian vessels than those associated with Britain at the time but the finish was crude by comparison. Though experts in marine archeology placed the boat to later than 1700BC in the prehistoric timeline Wright held firm to the belief it belonged to a much earlier period and was finally vindicated two months before his death at the age of 83 with the development of accelerator mass spectrometry which indicated a date as far back as 2030BC. Although it is 1,000 years younger than the proposed age of Avebury, it is also merely indicative of marine technology at the time (unless it was a prototype), so it's not possible to say for how long this particular design was in use or what preceded it.