A dome is a self-supporting structural element of architecture that resembles the hollow upper half of a sphere. Every dome in the world which was the largest dome of its time is listed below. The defining criterion is in each case the inner diameter of the largest circular cross-section of the dome. This list excludes dome-shaped structures that are not self-supporting such as The O2 in London which is 365 m (1,198 ft) in diameter and supported by masts.