 The most practical application is using a hill facing towards the equator (south in the Northern Hemisphere and north in the Southern Hemisphere), towards the aphelion (north) in the Tropics, or east just outside the Tropics.There is only one exposed wall in this type of earth sheltering, the wall facing out of the hill, all other walls are embedded within the earth/hill.This is the most popular and energy efficient form of earth shelter in cold and temperate climates.