Its history tells you why they are not exactly keen to let in all and sundry, although there is a covered public arcade around the base. It is built on the site of the former Baltic Exchange, the Edwardian shipping brokerage at the heart of the financial centre of the City of London. The Baltic Exchange was terminally damaged by a huge IRA bomb in April 1992. But just as in Manchester, where a similar bomb in 1996 devastated the city centre and led to a wholesale replanning exercise, the atrocity was turned to advantage. First came the calamity. Then came the opportunity. While the shattered buildings around were repaired and the City introduced its "ring of steel" security system, the site of the Baltic Exchange became an experiment in reinventing the skyscraper.

It is not only a post-IRA skyscraper, it is a post-al-Quaeda skyscraper. Swiss Re could conceivably have scrapped the design after the destruction of New York's Twin Towers - which, famously, they are involved in insuring. Construction of the Gherkin had scarcely begun when 9/11 took place. But the tower had anyway been designed for a city where - as the site's history makes plain - terrorist incidents are not unknown. The external triangulated steel structure, derived from the 1940s monocoque aeroplanes of inventor Barnes Wallis by way of the geodesic experiments of American inventor Richard Buckminster Fuller, is capable of absorbing prodigious amounts of damage without becoming unstable (structural engineers are Arup). So the rapid construction programme went ahead as planned, once Swiss Re had rechecked their total-evacuation plans for the building in the event of emergency. So when Foster talks of his admiration for his client, it is plainly heartfelt, and it is not only because they held their nerve. They wanted not just a tower, he says, but an eco-friendly, low-energy tower, one cooled by the wind, served by public transport, with car parking spaces only for the disabled. In energy terms is uses 50 per cent less power than a conventional building of similar size, drawing on the experience Foster's team gained in designing Frankfurt's Commerzbank tower in the 1990s. For most of the time it is naturally ventilated, with the spiral atria able to exploit the aerofoil-like differential between high and low pressure whatever direction the wind is coming from. Only during extremes of heat or cold will it be necessary to operate its zoned powered climate control systems.