The Great Wall is a series of fortifications built across the historical northern borders of ancient Chinese states. While construction began as early as the 7th century BC, the most famous sections were built by the Ming Dynasty (1368–1644). A surprising engineering fact is the use of 'sticky rice' in the mortar. Workers mixed slaked lime with a sticky rice soup, creating a chemical bond that made the mortar exceptionally strong and resistant to earthquakes and erosion. The wall served not just as a physical barrier, but as a communication system; smoke signals and fire were used to send messages between watchtowers, allowing news of an invasion to travel the length of the wall in a single day. Contrary to popular belief, it is not easily visible from the Moon with the naked eye, though it can be seen from low Earth orbit.