NEW YORK (CNN/Money) -
When our prehistoric ancestors took refuge in caves, they were seeking shelter from the elements and protection from the dangers of their surroundings.
Those reasons for occupying underground abodes still resonate with people. These days, many homeowners are thinking "green" (see "Green goes mainstream"), and are looking to cut energy use and conserve resources.
Underground dwellings are extremely energy efficient. Caves have year-round temperatures in the mid-to-upper fifties, making them a cinch to heat and unnecessary to cool.
Even subterranean homes that are not caves, strictly speaking, may benefit from similar insulating qualities. These include "berm" homes (which are built into a piled up mound of soil and stone) and "earthships" (which are dug into slopes, with tires then laid atop in a brick-like pattern and filled with compacted soil).
Builders orient many underground homes south and incorporate a wall of windows to capture the sun's winter rays more directly and exploit solar heat. That energy can also help keep excess humidity in check, resulting in comfortable, even toasty, homes.
Some occupants are more interested in the safety underground homes provide from life's dangers, like terrorist attacks, every day criminality, or nature's wrath. These people may convert decommissioned missile silos to residential use.
Whatever the style or structure, underground dwellings usually offer serenity from the cacophonous outside world. Underground bedrooms provide "the best sleep you ever had," says Bruce Francisco, who owns a former missile silo. "There could be a blizzard outside, but you feel totally peaceful."
Many underground homes occupy properties that sit amidst beautiful landscapes, which they take advantage of by incorporating at least some above ground elements in their designs.
Want to try out an earthship or cave living while you're on vacation? See "Unbelievable places to stay."