Petra covers approximately 100 square miles (260 square kilometers) and is famous for its rock-cut architecture and water conduit systems. Estimated to have been established in the 4th century BC, it was once a thriving trade center. The Treasury, one of its most iconic structures, stands at around 138 feet (42 meters) tall and is intricately carved into sandstone cliffs. Petra was declared a UNESCO World Heritage site in 1985 and attracts over 1 million visitors each year.