Petra, an ancient city built into the living stone, remained hidden away for centuries after the gradual decline of the Nabataean metropolis under Roman rule.
Built in what is now southern Jordan, the city was founded sometime after the death of Alexander, possible as early as 312 BC. The Nabataeans, who had been a herding people only a short time before, built this monumental city, and prospered during the Hellenistic era.
The city is only visible after entering the Siq- a three quarter mile long, and ten to twenty feet wide gorge that ends at the entrance the entrance to the Treasury (pictured below). The photo beneath the treasury is of the Siq.
Petra was built in a very inhospitable region, with only desert for miles, making the architecture all the more amazing. In fact, the site was voted to a new 7 Wonders of the World list in 2007.
The Nabataeans were described in the 4th Century BC as "living a rough and rural life, without cities or settled agriculture" by Greek historian Diodorus. They had changed significantly by the early 1st Century AD. During this time, Greek geographer Strabo described Petra as "a great city with an abundance of water, gardens, and expensive stone houses" (Auge and Dentzer, pg. 36).
While it would seem an unlikely place for this kind of undertaking, the location of the city was at a crossroads of trade routes between many of the great powers of the day. The Nabataeans grew rich collecting fees from the massive desert caravan trade, and provided travelers with food and water while traveling through the hot desert region.
Exactly where the Nabataeans came from, as well as how, and why, the site was settled is still debated. One thing that is understood is that the Nabataeans were heavily involved in the commercial trade of the surrounding regions. At its peak, the city was "a major caravan centre for the incense of Arabia, the silks of China and the spices of India, a crossroads between Arabia, Egypt and Syria-Phoenicia." Source
One of the most spectacular features of the ancient city must have been the way in which they brought water in for a population estimated to be between 10,000 and 30,000. They developed systems to help preserve seasonal rainwater, but that would not have been enough. To support the city, the Nabataeans built and maintained "an extensive hydrological system, including dams, cisterns, rock-carved water channels, and ceramic pipes." Source
In 106 BC, the Nabataeans were conquered by the Romans: "Petra became part of the Roman province of Arabia but continued to flourish until changing trade routes caused its gradual commercial decline."