shelters (Lättenstrasse in Dietikon). Possibly the most well known examples of modern earth sheltering in the English-speaking world are Earthships, the brand of passive solar earth shelters sold by Earthship Biotecture. Earthships are concentrated in New Mexico, USA, but are found less commonly throughout the world. In other areas such as the UK earth sheltering is more uncommon. Overall earth shelter construction is often viewed by architects, engineers, and the public as an unconventional method of building. Techniques of earth sheltering have not become common knowledge, and much of society is unaware of this type of building construction. Generally speaking, the cost of excavation, increased need for damp-proofing and the requirement for the structure to withstand greater weight relative to above grade houses means that earth sheltering remains relatively rare. In this respect, the Passive House (PassivHaus) energy performance standard applied to above grade airtight, superinsulated low carbon or zero carbon buildings has had much wider uptake in modern times. Over 20,000 buildings certified to PassivHaus standards have been constructed across Northern Europe. Some postulate that over time the reducing availability of building space, and the increasing need and interest for environmentally friendly housing will make earth shelters more common. There are an estimated 30-40 million Chinese living in such earth homes today still, mostly along the Yellow River in Northern China. There are mainly two kinds of earth homes in the area. The first type is pit cave dwellings. Usually, people dig a sunken courtyard, which is a rectangular underground pit about 10 meters deep, then arrange rooms dug off the main courtyard. The second type is dwelling space carved out of earth cliffs which are often south-facing, and the facades are sometimes faced with bricks or stone. They usually come with larger courtyards since it doesn't bother to excavate. Types Three main types of earth shelter are described. There is also great variation in the approach to earth sheltering in terms of materials used and expenditure. The "low tech" approach might involve natural building techniques, wooden posts and shed style roofs, recycling of materials, owner labor, hand excavation, etc. The relatively more high tech approach would be larger, using concrete and steel. While typically more energy efficient post construction, the high tech approach has higher embodied energy and significantly more costs. Bermed In the earth bermed (also termed "bunded") type, earth is banked against the exterior walls, sloping