At more than 13,000 miles long, the Great Wall of China is often called the longest feat of human engineering. It is more than 2,000 years old and took more than 1 million workers to build.

Adapted Trombe wall now used for both building heating, cooling. ScienceDaily . Retrieved June 2, 2025 from www.sciencedaily.com / releases / 2016 / 02 / 160224132930.htm ...