One of the big benefits of earthships is that they are made from recycled materials. The main construction materials of an earthship is used tires that are filled with dirt. The dirt-filled tires are than laid flat and stacked like bricks. Because the tires are round, there is lots of extra space between the column of tires. These spaces are filled with used cardboard. The tires and cardboard form the outside walls of an earthship. The inside walls between rooms in the earthship do not need to be as strong as the outside walls; therefore, the inside walls are made with old cans, bottles, and cement that are eventually covered and painted to look just like walls in a typical home.