The Pantheon in Rome has survived intact, illustrating classical Roman structure, proportion, and decoration. The main structure is a dome, the apex left open as a circular oculus to let in light; it is fronted by a short colonnade with a triangular pediment. The height to the oculus and the diameter of the interior circle are the same, , so the whole interior would fit exactly within a cube, and the interior could house a sphere of the same diameter. These dimensions make more sense when expressed in ancient Roman units of measurement: The dome spans 150 Roman feet); the oculus is 30 Roman feet in diameter; the doorway is 40 Roman feet high. The Pantheon remains the world's largest unreinforced concrete dome.