30 St Mary Axe (for obvious reasons also called The Gherkin) is a landmark 40-storey office building in the heart of London's financial centre, developed by Swiss Re and designed by architects Foster and Partners. The tower has a circular plan that widens as it rises from the ground and then tapers towards its apex which makes it appear less bulky than a rectilinear building of equivalent floor area. 30 St Mary Axe is London's first environmentally sustainable tall building. Among the building's most distinctive features are its windows, which open to allow natural ventilation to supplement the mechanical systems for a good part of the year. Whenever possible, recycled and recyclable materials have been specified throughout the building and innovative ways of reducing energy consumption have been implemented.