Rome's ruined yet impressive archaeological sites are landmarks in the city, which went through many changes during the thousands of years of its existence. These include the Coliseum, the Pantheon, the stadium, the arches of Titus, Septimius Severus and Constantine, The Temple of Antoninus and Faustina, Hadrian's mausoleum and many more. The historic center of Rome was declared a World Heritage site by UNESCO in 1990.