An ukdun is a kind of dwelling, originally devised by semi-nomadic and settled Yonggan people, and constructed with Earthbending. Similar structures are also in use by others in the Northern and Western regions of the kingdom where cold, dry winter climates prevail.
The ukdun is characterized by underground or mostly-underground construction and a circular plan, typically arranged concentrically. It also typically includes heating by means of a hypocaust and massive stone stove-bed, and a sheltered outer area for livestock. Ukdun vary in size and complexity, and can be made to accommodate quite large households. Semi-nomadic Yoggan build different designs for different seasons; Summer ukdun built by desert-dwellers may include an evaporative cooling system.
Among the Nara clan, Yonggan people exiled to the Hanwang desert, a heating system modification similar to a "rocket stove" is popular as an adaptation to limited fuel supplies. Dried badgermole dung is often used to supplement fuel, and can be burned without aesthetic concerns for the inhabitants because of the sealed nature of the heating system. The firebox is usually located in the livestock sheltering area for convenience and to avoid drawing cold air into the house for combustion. The hypocaust system may also be used for baking, in combination with a metal chamber to separate the combustion products from food, and to dry wet fuel.
Ukdun may or may not include a water well, depending on the local aquifer and the availability of potable surface water.
Most ukdun have one inhabitable floor, but multi-story ones do exist, particularly among wealthy settled Yonggan who do not wish to assimilate to conventional Earth Kingdom architecture.
Illumination is provided by skylights or an occulus in summer ukdun. Winter ukdun are lit by papered lightwells or glass skylights, but tend to be dark without artificial lighting. Unsurprisingly, the invention of the mantle lamp was welcomed by the Yonggan.
The term ukdun denotes specifically a home. However, similar communal buildings are also built by the Yonggan.