Ancient Merv, located in modern-day Turkmenistan, is a succession of cities, now ruins, which are perhaps best known for their association with the Silk Roads. The great Islamic city of Sultan Kala, at times capital of the eastern Islamic world, is the largest of the cities of Merv and was one of the largest cities in the world in the mid-12th century CE. Today, the site is one of the most complex and well-preserved urban archaeological remains in the world and it is part of the Ancient Merv World Heritage Site. The Ancient Merv Project is a collaboration between the Turkmenistan Ministry of Culture and the UCL Institute of Archaeology, and it systematically researches the archaeology of Merv.