|Behold, the ondol floor in my apartment.|
It makes sense that Korea would come up with this. Koreans spend a lot of time on the floor. Traditionally, this is where they slept, on futon-like mats called yo. Tables are often built low as well. In my apartment, I sit on the floor on flat pillows to eat. You'll see this in restaurants too. With all this living on the floor, it just makes sense that this is where the heat would be.
Traditionally, heat was dispersed by burning wood or coal in a furnace, and the smoke would be funneled under the floor, where it would warm stones. This served a dual purpose of cooling in the summer, as the stones tended to stay cool under the floor. Of course, the system wasn't perfect, with some spots being warmer than others, and leaking smoke causing carbon monoxide poisoning. In modern buildings, hot water is forced through pipes coiled under the floor.
This morning, I woke up around 5 to use the bathroom. Before falling back asleep I turned on the ondol. An hour later I was gently awoken by the heat rising through the bottom of my bed. It was perhaps the coziest I had ever been. When I stepped out of bed my feet were greeted with a warm floor and the heat continued into the kitchen, which has the same wood grain-like flooring.
However, there are drawbacks. There is no ondol in the bathroom. With no vent system to push hot air into the bathroom it stays cold all the time. Also, gas is used to heat the water in the ondol pipes and gas is expensive in Korea. My apartment is pretty big as well. I bought a space heater to try and cut down on ondol usage but it's just not the same. Also, it takes a while to heat up, and if you fall asleep without setting the timer you wake up in a sauna.
Unless, of course, that's what you're after.