The word ondol means "warm stone." Historians think it may have been invented in the 4th century, possibly even as far back as 5000BC.

If you visit one of Korea's historical villages or houses today, you'll probably get to see an original ondol. The floor in a traditional setup, also called gudul, is usually made of granite blocks, about 5 cm thick, supported by columns of brick or stone. The flue from a fireplace or kitchen stove is run between those supports, and that's what provides the heat. Believe it or not, this is essentially the same principle that modern high efficiency gas and oil furnaces and boilers use -- recovering heat from the flue gases. And Korea discovered it at least 1,700 years ago.

On top of the granite blocks is a concrete (originally clay) surface for smoothness and comfort under your ondol (futon) mattress. This was topped with oiled paper in the old days, but today it's usually vinyl.

A typical Korean house in the Choson Dynasty consisted of two rooms plus a kitchen. The flue from the kitchen stove ran under the floors of the other rooms. It wasn't unusual for the house to have extra fireplaces for more heat. Oddly, these were actually located outdoors, at or below ground level, with flues also routed under the floor. These were just for heating (not cooking). The chimneys were on the side of the house opposite the fireplaces.

The photo above shows a building at Ojukon, the Kangnung (Gangneung) home of Choson Dynasty statesman Yi Yul Guk (Yi I). The dark rectangular hole at ground level, just to the left of lower center in the photo, is the outdoor fireplace -- you can see the soot streak above it. The freestanding chimney for it is on the right side of the picture. It's lower than the roofline, which makes me wonder what kind of a draught it could've had.

Below we have an Ojukon kitchen. Here we see two ondol cooking fireplaces. Also check out the closeup of an ondol fireplace at Songyojong (The House of 100 Rooms). This fireplace is located below ground level, literally under the floor.

Almost nobody actually uses traditional ondols any more. Between the Japanese occupation and the Korean War, South Korea was pretty well stripped of its timber, and the government has been trying to reforest ever since. It's only been in recent years that they've allowed anyone to cut firewood at all. Instead, modern Korean homes and apartments have pipes in the floors, and gas or oil boilers circulate water through them. This produces about the same cozy (and I do mean cozy) effect as a traditional ondol, but without wood, and with better efficiency. The floors are usually covered with vinyl rather than carpet.

Today, you're not likely to see a genuine wood-fired ondol in use, except in historic settings (such as Hahoe Village, below).