Millennium Dome. Composite image of the Millennium (or Greenwich) Dome in London, England. At 320 metres across, it is the largest dome in the world. In the year 2000 it contained the Millennium Experience, a huge exhibition to celebrate the millennium. The 2 mm thick roof, made of teflon-coated glass fibre fabric, is supported by 70 km of steel cables attached to twelve 100 metre high masts.