Underground living

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Underground living refers simply to living below the ground's surface, whether in naturally occurring caves or in built structures.

Besides its obvious novelty, underground living offers additional benefits when compared to living in traditional buildings, such as a nearly constant comfortable temperature without the need for additional insulation, quiet, resistance to hurricanes, tornadoes and most weapon systems and the unobtrusiveness of such buildings on the landscape. One of the greatest advantages is energy efficiency. The stable subsurface temperature of the earth saves around 80% in energy costs^{[citation needed]}. If married to solar design, the energy bill can be completely eliminated. Additionally, the noise insulation of the surrounding earth makes underground homes exceptionally quiet, and with a smaller surface area, fewer building materials are used.

However underground living can be easily affected by flooding and sometimes special pumping equipment is necessary. Underground living has been both a feature of fiction, such as Hobbiton as described in the stories of J. R. R. Tolkien and The Underground City by Jules Verne. It is also the preferred mode of housing to communities in such extreme environments as Australia's Coober Pedy, Berber caves as those in Matmâta, Tunisia, and even Amundsen-Scott South Pole Station. Underground living is even being considered for the design of a future base on Mars.

Often, underground living structures are not entirely underground, typically if they are exposed on one side when built into a hill. This exposure can significantly improve interior lighting, although at the expense of greater exposure to the elements.

[edit] Structures

There are various ways to develop structures for underground living.

Caves (Natural) have been used for millennia as makeshift shelter.
Caves (Constructed) are a common structure for underground living. Although the tunnelling techniques required to make them have been well developed by the mining industry, they can be considerably more costly and dangerous to make than some of the alternatives. On the plus side, they can be quite deep. One examples would be the town of Coober Pedy in Australia, built underground to avoid the blistering heat of the Outback.
Earth Berm structures are essentially traditional homes that have then been buried, typically leaving at least one wall exposed for lighting and ventilation. However, because they are to be buried, the structures must be made of materials capable of surviving the increased weight and moisture of being underground.
Rammed Earth structures are not truly underground, in the sense of being below grade or buried beneath a berm. Instead, they are structures made of tightly packed earth, similar to concrete but without the binding properties of cement. These structures share many properties with traditional adobe construction.
Culvert structures are a very simple approach. Large precast concrete pipes and boxes a few meters across are assembled into the desired arrangement of rooms and hallways onsite, either atop the existing ground or below grade in excavated trenches, then buried. This approach can also be referred to as Cut and Cover.
Urban underground living is so common that few even think of it as underground. Many shopping malls are partially or totally underground, in the sense that they are below grade. Though not as exotic as the other underground structures, those working in such urban underground structures are in fact living underground.
Shaft structures are perhaps the most exotic of all underground living, more like Zion in the film The Matrix than a hobbit hole. For example, Japan's proposed Alice City project would incorporate a very wide and deep shaft, within which would be built levels for habitation, all looking in toward a hollow core topped with a huge skylight.
PSP construction, as popularized by Mike Oehler (the $50 and up underground house book) involves using a simple system of posts, shoring, and polyethelyne plastic sheeting (for waterproofing) to create a frame underground. The walls are later backfilled, and the roof is covered with a conservative amount of soil to create a housing structure which is completely underground.