I've heard about ondol, Korea's traditional heating system. How does it work? Has Korea exported ondol?

Ondol is a heating system hidden beneath the floor. There are different theories on how old it is, but Chinese records state that during Goguryeo (37 B.C.-A.D. 668), an L-shaped ondol that provided partial heating to the room floor was common. It evolved into a full-room ondol in the Goryeo period (918-1392). By the end of Goryeo, the ondol had spread to the entire Korean Peninsula.