The Great Wall is under attack. Not by hordes of invading Mongols, but by man-made ecological catastrophe. While most of us have a mental image of a solid, stone wall stretching off into the distance, a product of stock photos and tourist itineraries, the Wall is actually a complex work, built over thousands of years over innumerable building campaigns. Substantial sections were built in more fragile mud brick. Now, although not as sturdy as stone, this in itself is not necessarily a problem for conservation. After all, much of the wall has survived for millennia. However, unrestrained farming in north-central China has resulted in conditions approximating the Dust Bowl of the 1930s. This is bad not only for the inhabitants of the region, but for monuments such as the Great Wall. Already more than 25 miles of wall have been completely eroded by sandstorms in the last 20 years, but much of the rest of the 'standing' wall has been reduced to little more than stubs on the landscape, as the picture above shows. Archaeologists are trying to protect the remaining sections by burying them in dirt, but this can only be a stopgap measure. At the current rate of degradation, this entire portion of the Wall will be gone in 20 years.