At least, that’s what they are called in France’s Loire Valley. Here in the United States, the most famous troglodyte residence, or cave home, is occupied by the Sleeper family, of Festus, Missouri, and their sandstone home – formerly both a concert hall and a roller skating rink – offers modern-world amenities with primeval space heating and cooling.

The heating and cooling, or absence thereof, is in fact the main reason many people choose cave, or earth-sheltered homes. At six feet below the surface of the earth, the temperature is a consistent 45 degrees to 75 degrees (Fahrenheit).

Earth is not a good insulator. It is, however, a good temperature modulator. In Phoenix, for example the temperature at six feet below ground is about 60 to 80 degrees. At ten feet, it is 68 to 72 degrees. In Minnesota, a cooler climate zone, it is 42 to 55 degrees, but this is a far cry from the outside, where temperatures range wildly from 95 in the summer to -25 in the winter.

By relying on such consistent temperatures, cave homes (and earth-sheltered homes) need very little heating and cooling. For the Sleepers, this means all the luxuries of a modern American home without the cost of heating and cooling, which is the largest part of a residential homeowner’s budget.

The Sleeper’s home is the only known cave home in the U.S., but not the only cave complex seeing active use. That honor goes to Hunt Midwest, whose underground complex of limestone caves near Kansas City offers a friendly ecological footprint to such businesses as Paris Brothers food service company, Underground Vaults & Storage Co., and Mail Print.

While cave homes “catch on” in the U.S., the city of Granada, in Spain, continues to enjoy its legacy of cave homes. There, however, the caves are largely manmade, dating from a thousand years ago, and – in some neighborhoods like Benalua and Sacromonte – represent the bulk of urban shelters.

In addition to moderating temperatures, cave homes and earth-sheltered homes also offer sound-proofing against the (sometimes frenetically noisy) world outside. Add windows in front like the Sleepers did for daylighting, some solar panels for electricity and a solar thermal system for hot water, and cave homes can achieve a near zero or net zero energy footprints.

This, net zero, is the goal of modern, eco-friendly architects moving into a new era of reduced fossil-fuel availability and continued climate change – the two hinges on which the door to tomorrow swings open or closed.