Here's an up close look at one.
Wikipedia do the 'splaining:
An Earthship is a type of passive solar house made of natural and recycled materials. Designed and marketed by Earthship Biotecture of Taos, New Mexico, the homes are primarily constructed to work autonomously and are generally made of earth-filled tires, using thermal mass construction to naturally regulate indoor temperature. They also usually have their own special natural ventilation system. Earthships are generally Off-the-grid homes, minimizing their reliance on public utilities and fossil fuels. Earthships are built to utilize the available local resources, especially energy from the Sun. For example, windows on sun-facing walls admit lighting and heating, and the buildings are often horseshoe-shaped to maximize natural light and solar-gain during winter months. The thick, dense inner walls provide thermal mass that naturally regulates the interior temperature during both cold and hot outside temperatures.Here's an exposed wall that illustrates the construction method.