Underground. The real underground earth shelter (also known as a "chambered" or "subterranean") earth shelter is a dwelling in which the ground has been dug and the structure is built below grade. They may have an atrium or courtyard built in the center of the shelter to offer appropriate lighting and ventilation. These dwellings were very popular in Europe and North America during the late 1940s and early 1950s. There are also underground cabins which are smaller than true earth shelters but large enough for one person to live in comfortably. These are usually made out of wood and can be found in forests around the world. There are even some that are made from concrete! Underground houses have many advantages over conventional housing. They are more energy efficient because they avoid heating and cooling empty space. They are also better for privacy because you cannot see them from the outside. And they allow for natural light and ventilation without using electricity for lights or air-conditioning. The underground house movement was started by Jens Thiis in Denmark. He called his version an "earthship." The first one he built had no flooring and only had two walls and a roof above it. It looked like a big ball buried in the ground. But since then, people have added rooms, floors, and exterior walls to their underground homes.