BUSTED: At more than 3,300 km (2,000 mi) long, the Great Wall of China is one of the most impressive structures on Earth. Despite its size, this engineering marvel is very difficult to see from space. That’s because “its color and texture blend into the surrounding terrain,” explains Tim Newman, a program coordinator for the U.S. Geological Survey (USGS).
The Great Wall is difficult to spot in high-resolution photos of Earth taken by USGS satellites, which orbit 700 km (440 mi) above the planet’s surface. Even astronauts aboard the International Space Station, just 400 km (250 mi) above the ground, report that the
Great Wall is visible only with the help of magnifying camera lenses.
Other human-made landmarks are easier to see. For example, says Newman, “China’s Three Gorges Dam is visible not only because it’s large but because its white color stands out against the blue water.” Even smaller structures like roads and crop fields can stand out clearly when their surroundings are a different color, he says.