https://en.wikipedia.org/wiki/Ondol#History

> Ondol ..or gudeul ... in Korean traditional architecture, is underfloor heating that uses direct heat transfer from wood smoke to heat the underside of a thick masonry floor. In modern usage it refers to any type of underfloor heating, or to a..........

> Use of the ondol has been found at archaeological sites in present-day North Korea. A Neolithic Age archaeological site, circa 5000 BC, discovered in Unggi, Hamgyeongbuk-do, in present-day North Korea, shows a clear vestige of gudeul in the excavated dwelling