The Great Wall of China is a 6,352 km (3,948 miles) long Chinese fortification built over a span of nearly 2,000 years, in order to protect the various dynasties from raids coming from areas in modern-day Mongolia and Manchuria. Along most of its arc, it roughly delineates the border between North China and Inner Mongolia. Although it is commonly said that the wall is visible to the naked eye from space, it is more accurate to say that it is visible under favorable viewing conditions, if one knows exactly where to look.