When it comes to sustainable housing, there’s often a focus on technology, with homes that feature solar panels, energy-efficient glass, and a mobile form factor. But our ancient ancestors may have already come up with the best form of shelter: building homes partially underground.

Earth-sheltered buildings are typically built into hillsides.

Sometimes earth is built up around the structure, but usually these homes are just built into an existing hill or earth mound.

They’ve been a part of human history for a long time.

Dwellings like this provided more warmth than caves, making them a necessity in harsh northern climates.

These homes stay warm through the magic of thermal mass.

The ground absorbs and retains heat, meaning that the inside of an earth-sheltered home maintains a fairly consistent temperature.

Owing to their solid construction, they’re not prone to drafts.

This is because, depending on the design, very little of the structure is exposed to the outside.