<human>:  Can you tell me some facts about the Great Wall of China?
<bot>: The Great Wall of China is a series of fortifications that were built to protect the borders of the Chinese Empire. It is one of the most famous sites in the world, and is the longest wall in the world, running for an estimated 8,850 kilometers (5,500 miles). It was constructed over many centuries, with the earliest sections being built as early as the 7th century BCE. The wall was initially built using earth, stone and other materials but was later reinforced with brick and granite. Construction continued until the 16th century, during the Ming dynasty. Today, many sections of the wall are open to visitors, providing them with an opportunity to appreciate the long and rich history of the Chinese people.