Petra, originally known to its inhabitants as Raqmu, is a historic and archaeological city in southern Jordan famous for its rock-cut architecture and water conduit system. Established possibly as early as the 4th century BC as the capital city of the Nabataean Kingdom, it is often called the 'Rose City' because of the color of the stone from which it is carved. The Nabataeans were master hydrologists, creating an elaborate system of dams and cisterns that allowed a city of 30,000 people to thrive in the middle of the desert. The most famous structure is Al-Khazneh (The Treasury), which features a façade carved directly into the sandstone cliff face. Access to the city is through a narrow gorge called the Siq. Petra was a vital crossroads between Arabia, Egypt, and Phoenicia, serving as a major hub for the trade of frankincense, myrrh, and spices.