Cob is an ancient building material that's basically wet earth and straw mixed together and rolled into loaf-sized pieces or cobs. The mixture is very similar to clay, and what makes cob houses unique and beautiful is the organic shape. Instead of assembling and covering a frame, builders stack cobs, then use the same clay-like material to mold the walls by hand [source: Liloia]. The result is a structure with curving lines instead of sharp angles, and many cob structures include lots of fun, built-in features like shelves and hooks molded right into the walls. Some of these homes even feature built-in furniture, like couches and tables, molded from cob.
Because cob is made from natural materials, it has a very low environmental footprint compared to other building materials like concrete. Cob doesn't have to be made in a factory and shipped across the country [source: Liola]. And you can find mud and straw very close to the building site.
Cob is so simple to work with that many people build cob houses themselves. There are even groups, like Natural Building Network, that offer workshops on how to build your own cob house.