This 'living skin' has protected the Great Wall of China for 2,000 years

It’s known as a ‘biocrust’. It’s made of lichen, moss and bacteria along with tightly bound soil particles. Biocrusts are no more than a few centimetres thick but they do a vital job preserving the wall, scientists have found. Making it stronger and less porous, stopping water and salt from seeping in and buffering against the freeze-thaw cycle in winter.