There are several more photos on the Flickr page. This photo shows a detail of rammed-earth construction. It is a method of construction that strikes a compromise between using local materials, such as adobe (cheap, with minimal transportation needed, but very slow to build and very labor-intensive), and premade materials such as cinder block (cost more, takes a lot of energy to ship, but go up fast and with less labor.) The walls are 18 inches thick, so it has immense thermal mass. It is also very quiet inside. And bulletproof, literally.