Dugout and semi-dugout homes, which are completely or partially built underground, have been constructed on every inhabited continent for millennia. The move makes sense, especially in areas with little available building material: what better way to stay warm and dry if timber, thatch and loose stone aren't readily available?
The early underground homes in Coober Pedy, Australia, are a prime example of form following function. Opal miners digging in the region's sandstone would move into their mine shafts after pulling out as much of the gem as they could. Over time, entire communities grew out of the abandoned mines, complete with churches and community gathering spaces. The communities grew even larger as non-miners decided to build their own underground homes, using tunneling machines to create dwellings that surpassed the original hand-dug shafts in their beauty and functionality [source: Outback Australia Travel Guide].
Today, opal mining is no longer practiced in Coober Pedy, but homeowners continue to build and expand underground homes in the stable, insulating sandstone. These are perhaps the most extreme example of a green-building technology -- using the earth itself as walls, roofs and floors -- that has been in use since before recorded history. With the modern homes of Coober Pedy, however, the technique has been refined to an architectural art.