Mohenjo-daro was built in the 26th century BCE. It was one of the largest cities of the ancient Indus Valley Civilization, also known as the Harappan Civilization, which developed around 3,000 BCE from the prehistoric Indus culture. At its height, the Indus Civilization spanned much of what is now Pakistan and North India, extending westwards to the Iranian border, south to Gujarat in India and northwards to an outpost in Bactria, with major urban centers at Harappa, Mohenjo-daro, Lothal, Kalibangan, Dholavira and Rakhigarhi. Mohenjo-daro was the most advanced city of its time, with remarkably sophisticated civil engineering and urban planning. When the Indus civilization went into sudden decline around 1900 BCE, Mohenjo-daro was abandoned. The ruins of the city remained undocumented for around 3,700 years until R. D. Banerji, an officer of the Archaeological Survey of India, visited the site in 1919–20, identifying the Buddhist stupa (150–500 CE) known to be there and finding a flint scraper which convinced him of the site's antiquity. This led to large-scale excavations of Mohenjo-daro led by Kashinath Narayan Dikshit in 1924–25, and John Marshall in 1925–26. In the 1930s, major excavations were conducted at the site under the leadership of Marshall, D. K. Dikshitar and Ernest Mackay. Further excavations were carried out in 1945 by Ahmad Hasan Dani and Mortimer Wheeler. The last major series of excavations were conducted in 1964 and 1965 by Dr. George F. Dales. After 1965 excavations were banned due to weathering damage to the exposed structures, and the only projects allowed at the site since have been salvage excavations, surface surveys, and conservation projects. However, in the 1980s, German and Italian survey groups led by Dr. Michael Jansen and Dr. Maurizio Tosi used less invasive archeological techniques, such as architectural documentation, surface surveys, and localized probing, to gather further information about Mohenjo-daro. A dry core drilling conducted in 2015 by Pakistan's National Fund for Mohenjo-daro revealed that the site is larger than the unearthed area.