More than 30 years after it disappeared from Dublin's streetscape, Nelson's Pillar is still making history. Paul Cullen unravels a tangled tale of buried treasure and the pillar's 'time capsule'

The revelation last autumn that a "time capsule" had been discovered in the foundation of Nelson's Pillar wasn't just big news in the archaeological world. In media terms, it was a peach of a story, powered by the vivid image of a 200-year-old treasure trove in which the merchants of Dublin may have stored away the essential items of their life and culture for posterity.

The anticipation that accompanied the opening of the granite box found at the pillar was exceeded only by the widespread disappointment that followed when it was found to be empty. The "time capsule" was a dud.

Subsequent research has placed this discovery in its true context in the rich tapestry of Dublin history, far removed from the soundbite notion of a "time capsule" filled with lost treasure.

The story begins on February 15th, 1808, when the foundation stone was laid at the centre of the monument. In the period immediately after the Act of Union, Dublin was undergoing a period of relative calm. The city's prominent merchants, among them Arthur Guinness and the La Touche banking family, came together to commemorate Admiral Nelson and his victory in the Battle of Trafalgar. The group included a number of Catholics, including one Denis Thomas O'Brien, a cotton manufacturer from Balbriggan.

A brass foundation plaque inscribed with the names of the largest contributors was laid in a cavity cut into the granite foundation stone. The event was recorded in the Freeman's Journal: "When the stone was placed, a Brass Plate was handed to his Grace [Charles Lennox, Duke of Richmond and lord lieutenant at the time] by the Architect, Mr Johnston. After perusing the inscription on it for a few minutes, his Grace placed it on a bed made in the foundation stone. Some melted rosin was placed around it, and another stone placed over it. Thus concluded the ceremony."

Over the years, Dublin Corporation made various attempts to remove the pillar, but the end came finally on the night of Easter Monday 1966 when republicans blew it up. Two days later, army engineers blew up the rest of the stump.

The foundations were left untouched until last September, when the archaeologists were sent in, in advance of construction work for the controversial Monument of Light.

This team, under Franc Myles, uncovered the remains of the first buildings on the site, including the cellars of narrow, three-bay Dutch Billys dating to the last decade of the 17th century. This was the first time archaeologists had been able to trace the city's development north-eastwards from its medieval core. However, most interest centred on the pillar and its foundation stone.

Here, the archaeologists found exactly what they expected to find - a cut granite block about 2.5 feet long and 1.5 feet wide. In it had been cut a cavity measuring 1.5 feet by 10 inches, and at the base of this cavity there was a second, smaller cavity. The cavities and plaque were covered with a distilled pine resin called rosin, which acted as a form of sealant, and a limestone lid had been placed in the larger cavity.

The box was taken to the museum's conservation laboratory at Collins Barracks. Staff proceeded carefully with the removal of the lid and plaque by hand. The cavity below was found to be empty.

It is hard to understand now how the "time capsule" story gained currency. The contemporary accounts in the Freeman's Journal and other newspapers are quite precise and, as it turns out, accurate. They make no mention of hidden coins or documents.

As Franc Myles's report on the excavations makes clear, the second cavity was used not for storage but for lowering the stone into position. Besides, it is little larger than the size of a pack of cigarettes.

What evidence there is for the existence of a treasure trove seems to come from a book published 10 years after the foundation stone of the pillar was laid. In their History of the City of Dublin, Warburton, Whitelaw and Walsh recorded the deposition of "various coins" in a recess covered by a brass plate. However, as Myles notes in his report, this book has been shown to be unreliable, to the extent that the authors credited the wrong person with the design of the monument.

Nonetheless, when the pillar was blown up in 1966, their claims were revived, and rumours swept Dublin of the existence of a hoard of coins in the foundations, which were not recovered in the rubble. There was undoubtedly some pilfering before the site was cleared up, and Nelson's sword and head disappeared; the sword is still missing.

Two years ago, a number of coins from 1805 and 1806 were put up for auction in Dublin and advertised as being "apparently from under the foundation stone of Nelson's Pillar". However, Myles is unconvinced, noting that it was unlikely that low-denomination coins such as farthings and halfpennies would have been deposited in such a major civic monument.

While some still cling to the notion that the contents of the casket were robbed, the reality is more prosaic. There was no treasure trove, and it was clear before excavations began that none would be found. The "time capsule" story was just that - a good story.

The foundation stone and other items relating to Nelson's Pillar are currently on exhibition at the National Museum in Collins Barracks