The more thermal mass (weight that can absorb heat) a building has, the more it will average out the temperature around it. For example, in a cave underground in England the temperature is an average of the yearly temperatures – around 10 degrees C. An earth shelter takes advantage of this by having the building partially buried in the earth or with earth mounds up the sides.
Earth shelters are just one of the types of buildings that Idp Search architects produce but as well as new builds they crucially also do retrofitting – altering an existing building to make it less wasteful. One project they have been using is called Greening the Box. You can see an example of their work which is in High Wycombe.