Rammed earth is a technique for building walls using raw materials such as earth or gravel. Construction starts with a temporary wooden framework which is filled up by a mixture of damp earth (sand, gravel, and clay). In ancient times, additions were made in the form of lime or animal blood to stabilize the material. Once the material is poured into the mould, it's compacted to half of its original height. After this, a new layer is added and the process is repeated until the wall is completed.
The technique was first used in the region around the Yellow River in 5000 BC. By 2000 BC it was commonly used in walls and foundations around the country, and even sections of the Great Wall of China utilized it.