The Pantheon was originally built in 27 BCE as a temple honoring all gods, with the name coming from the Greek words for "all" and "god." It was rebuilt in 126 CE under Emperor Hadrian and has a circular structure under a dome opening to the sky, making it the world's largest unreinforced concrete dome nearly 2,000 years later. It was later converted to a church in the 7th century CE and remains one of the best preserved buildings from ancient Rome.