Earthships incorporate many natural and reclaimed materials in their construction. Tires are the perfect form for a rammed-earth brick. There’s no shortage of used tires—at least 2.5 billion are currently stockpiled in the United States, with 2.5 million more discarded every year. Tires can be seen as a globally available “natural resource.” Other materials such as cans and bottles are optional, although bottle brick walls have become an iconic decorative feature of many Earthships. All interior walls are packed out between the tires and plastered with adobe mud. Mud can also be used for floors, and reclaimed wood and metal are often used.