Dating back to the second century BC, the Nabataean archaeological site, also known as Madain Saleh, has long been hidden from foreign visitors in this ultra-conservative kingdom that rarely opens up to tourists.
Saudi Arabia is thought to have been wary of archaeologists and scientists seeking to study its ancient ruins for fear their findings could shine the spotlight on pre-Islamic civilisations that once thrived there.
In recent years, however, Saudis have increasingly ventured to these sites and the authorities are more tolerant of their curiosity.
Described as the largest and best preserved site of the Nabataean civilisation south of Petra in Jordan, Madain Saleh is the first Saudi archaeological site to be inscribed on UNESCO's World Heritage List.
It lies 320 kilometres (200 miles) north of Medina, the Islamic holy city of western Saudi Arabia, and extends for some 15 square kilometres (six sq miles).
According to UNESCO, it includes 111 tombs, most of which boast a decorated facade, cave drawings and even some pre-Nabataean inscriptions.
It also boasts intricately designed water wells that serve as a prime example of the Nabataeans' architectural and hydraulic genius ( via google.com ).