An extensive project to stabilise Silbury Hill has reached completion, following vital works to repack voids and an historic tunnel running through to its centre which had threatened the structure of the Neolithic mound. The success of the project will be marked today with the sowing of grass and flora seeds on the top of the Hill by all those involved in the work.

The £1.66 million conservation project by English Heritage archaeologists and contractors Skanska civil engineering came about after a hole appeared in the summit of the Hill in 2000, revealing the need for extensive stabilisation work including work inside the 85-metre-long tunnel, which was dug to its centre by the archaeologist Professor Richard Atkinson in the 1960s.

The programme of works proved far more complex than first anticipated, with heavy rainfall in the summer of 2007, causing further collapses in the tunnel and dangerous tunnelling conditions for the team. However, Skanska and English Heritage devised a new, safe tunnelling method and a revised backfilling methodology in order to continue the work, and have now successfully completed the project, having filled in the tunnel and voids in the hill, as well as the voids in the hilltop and hillside, by repacking them with chalk, sealing and stabilising the mound for the long term.

Mark Kirkbride, Skanska Project Manager, said: "Over the last 12 months the project has been a unique and complex engineering challenge and we have found Silbury to be an incredible feat of construction. The tunnelling work has been very unusual and the conditions at times difficult but, through working successfully with English Heritage, we have achieved all of the original aims of the conservation works. We are confident that the Hill will now stand safe for future generations to marvel upon."

The project has not only successfully ensured the stability of the 4,400-year-old Hill for the long term, but also allowed the team to carry out further investigations into the age and possible use of the mysterious mound.

Dr Amanda Chadburn, Inspector of Ancient Monuments at English Heritage said: "Silbury Hill is one of the key monuments of the Stonehenge and Avebury World Heritage Sites, and is of global importance. It is recognised as a masterpiece of human creative genius and is the largest prehistoric mound in Europe. It is proof of the creative and technological abilities of prehistoric peoples to conceive, design and construct features of great size and complexity.

"Given this importance, it was essential that the voids and tunnels within the Hill were backfilled to prevent further archaeological damage to this unique monument. English Heritage and Skanska have worked closely together to achieve this goal, and despite the many unexpected difficulties faced by the Project Team, we are all delighted to see that the Hill is once again secure for the long-term."

As a result of the research undertaken by English Heritage, archaeologists discovered signs that the summit of the Hill may have originally been more domed in shape, and was truncated later on, in Saxon or Norman times, to create the current shape. The flat top could have been created in order to house a building. The team discovered a series of medieval postholes and two arrowheads, suggesting a defensive or military function, such as a lookout post or signal station.

Because the tunnel cut through many of the Hill's construction phases, archaeologists were also able to take advantage of a rare opportunity to make detailed records and investigations along the tunnel, which will enable them to date more accurately the phases of the construction of the Hill and give a clearer understanding of its original purpose.

Atkinson's work in the '60s identified three phases of the hill: Silbury I - an organic mound; Silbury II - a chalk mound with a quarry ditch; and Silbury III - the final mound you can see today, which buried the earlier ditch and was itself constructed of chalk quarried from the surrounding ditch. The recent work by archaeologists, led by Jim Leary, has, however, shown this to be a very simplistic model of the Hill. The mound is far more complex, and can be seen to grow through many small construction events, rather than a few grand statements, as believed by Atkinson. The implications this has for understanding the reasons behind the mound, and for Neolithic society generally, are only just beginning to be understood.

The part of the tunnel which cut below the original Neolithic ground level revealed the earth's natural geology with bright white Cretaceous chalk which is millions of years old. This is overlaid by clay with flints deposited during the Ice Ages, and then the Neolithic ground surface. During excavations, archaeologists found parts of antler picks as well as animal bones and flint cutting tools. These remains will help to date the phases of construction much more accurately and could radically alter our understanding of the Hill's purpose.

Organic remains such as molluscs and insects have also been found.

The unusual burial conditions within the centre of the Hill mean that they have been excellently preserved. With analysis, English Heritage experts will be able to create a complete picture of the Neolithic landscape including the type of vegetation, the climate, and how the land was managed by prehistoric people for grazing, arable and woodland use.

Stephanie Payne, Conservation and Land Management Adviser for Natural England in Wiltshire said: "The successful completion of this important project will ensure the conservation of unique archaeological features and important wildlife habitats at Silbury Hill. Natural England has worked closely with English Heritage and Skanska to ensure that the work minimised any impact on the environment, which contains many species including the uncommon Round Headed Rampion, Devil's Bit Scabious, Saw Wort and Horseshoe Vetch."

The iconic artificial Neolithic mound is also a nationally important wildlife site (a Site of Special Scientific Interest) for the species-rich chalk grassland which has developed since the mound was created. Special care was taken during the project to minimise the area affected. Where some disturbance was unavoidable turf was removed by hand, watered and carefully replaced at the end of the project.

Local wildflower seed collected either from the Hill itself or other similar chalk downland sites in the local area will be used to reseed key areas where required, to ensure there is no detrimental long-term visual or environmental impact. In other areas, bare chalk will be left to colonise naturally. This has been shown to be effective in other sites, especially where there is a rich seed source adjacent.

A presentation on the archaeological findings and the dating of the Hill by the English Heritage team will take place later in the year.