Built by Poelaert between 1866 and 1883, this building otherwise known as the "Mammoth" is enormous - 26,000 m² of floor space, exterior measuring 150 by 160 metres, a 42 metre high entrance and a dome that stands nearly 100 metres high. The main entrance is framed by a vast peristyle opening onto the Room of Lost Steps, covered by a cupola around which a gallery runs, which is accessed by enormous staircases.