The largest "ondol" heating system dating from the Balhae Kingdom has been unearthed in a nearly intact state in Russia's Maritime Province, confirming the kingdom to have been a Korean settlement.
Ondol, literally "warm stone", is an under-floor heating system where flues carry hot gases below the living space. They were a distinct feature of Korean dwellings and are not found in the remains of Chinese, Khitan or Jurchen homes.
The discovery proves not only that Balhae was a successor state to the ancient Korean kingdom of Koguryo, but also defeats the logic of China's recent "Northeast Project", which says Koguryo and Balhae were simply autonomous Chinese frontier districts.
The Koguryo Research Foundation and Russia's Institute of History, Archaeology and Ethnology of the People of the Far East, which are conducting joint excavations at a site in the Russian town of Kraskino, announced Thursday they confirmed remains of ondol pipes 14.8 m in length presumed to be from the 10th century, toward the end of the Balhae period.
The trace of the U-shaped ondol pipe which points toward the southwest, is 3.7 m. wide to the west, 6.4 meters to the north and 4.7 meters to the east, and is 1-1.3 m wide. Prof. Evgenia Gelman of Far-Eastern State Technical University, who unearthed the remains, said the discovery clearly showed Balhae to have been a successor state to Koguryo.