First underfloor heating

The earliest underfloor heating systems were developed 3,000–4,000 years ago in what is now northern China, Korea and the Russian Far East. These systems (known as oang or dikang in China and ondol or gudeul in Korea) have a small hearth with a horizontal flue that ducts hot smoke under the floor of a home or under a raised living area. These systems were developed from an earlier "baked earth" system, in which a dense clay floor would be heated by a surface-level hearth, storing heat after the fire was damped down. The oldest example of an ondol dates from around 1000 BCE, and was discovered by archaeologists in Unggi, Hamgyeongbuk-do, in present-day North Korea.