Two men walk in front of Qaṣr al-Farīd, or "the Lonely Castle," in Mada'in Saleh, Saudi Arabia. The structure is the largest and one of the best-preserved out of more than 100 tombs on the site, all of which date from approximately the 1st century BC to the 1st century AD during the pre-Islamic Nabataean Kingdom. In 2008, Mada'in Saleh—also known as Hegra or Al-Hijr—became Saudi Arabia's first UNESCO World Heritage Site.