Almost every traditional architecture in the world consists of putting material on top of ground. Wall, beam, roof. There is one large exception, and it affects tens of millions of people: in northern China the house is not raised, it is removed.
They are called yaodong, and they are dwellings excavated horizontally into a hillside or into the wall of a sunken courtyard. There are no foundations, no structure and no building materials. There is a hole, and what makes it possible is the nature of the ground.
01 A rock that is not a rock
The material is called loess and it is not rock: it is silt, mineral dust carried by the wind and accumulated over millions of years. Deposition on this plateau began 2.4 million years ago, and the material came from the Gobi and the neighbouring deserts.
The thickness is what surprises. Near the Liupan mountains the deposits reach between 200 and 300 metres, and beside the Yellow River some 100. The full plateau covers 635,000 square kilometres, 6.6 % of China's surface.
And it has one decisive property. Loess is soft — you can dig it with hand tools — but cohesive in the vertical: it holds a plumb cut without crumbling. That combination is rare and it is exactly what you need to live inside an embankment.
02 Three ways to dig a house
There are three types and all three can be told apart at a glance. The commonest is the cliffside kind: a series of caves opened in the face of a bank, rectangular in plan with a round-arched vault, lined up on the same level like the arches of a porch.
The second is the sunken courtyard. When the ground is flat and there is no bank, a square courtyard several metres deep is dug first and then the rooms are opened in its four walls. From above all you see is a rectangular hole in a field.
The third is the built-arch kind, for ground where digging is impossible. A vault is raised with material and then covered with earth, so that the result behaves like a cave even though it is not one.
03 Cool in summer for nothing
The reason this kind of dwelling has lasted three thousand years is not a shortage of materials: it is thermal behaviour. Several metres of earth above and around work as a mass that damps the outside temperature swings.
The effect is described directly: in summer the interior behaves like an air-conditioned room, and in winter it needs hardly any heating. In a climate where Xi'an records average highs of 32.8 degrees in July and average lows of −4.4 in January, that is not a comfort detail.
And in the province's north the range is wider still. A house that solves both extremes on its own, with no equipment and no fuel, is a technical solution hard to improve on with the means of a rural economy.
04 Thirty or forty million people
The scale of the phenomenon is the part almost nobody expects. In the early 2000s, between 30 and 40 million people lived in yaodong in northern China. That is a figure on the order of a large European country's population.
They are spread across five provinces: Gansu, Shaanxi, Shanxi, Henan and Ningxia, that is, the extent of the Loess Plateau. The country's highest concentration of cave dwellers is in the Qingyang region of Gansu.
The figure, however, is falling fast. Urbanisation and the move to conventional housing have emptied many groups, and much of what remains is used as storage or stands abandoned. It is a living architecture that is going out.
05 The problem with digging in dust
It is worth saying what does not work, because it is part of the story. Loess holds up well vertically and dry, but it is sensitive to water and responds badly to shaking. An excavated dwelling has no structure to give: the roof comes down.
The precedent is the gravest in recorded history. The Shaanxi earthquake of 1556 killed some 100,000 people, and many of those deaths are attributed to yaodong collapsing. Resistance varies a great deal with soil type and location.
It is the flip side of the very material that makes the dwelling possible. Soft, cohesive silt is dug without effort, and for the same reason it forgives neither water infiltration nor a strong movement.
06 How to look at it
The first practical consequence is that you have to know what to look for, because they are invisible from the road. A group of cliffside yaodong looks like a retaining wall with doors, and a sunken-courtyard one looks like nothing at all. What gives an inhabited slope away is the row of vaults at the same height.
The second is that the province's north is the place. Yulin and Yan'an are in the middle of the Loess Plateau, and there the landscape of hills furrowed by gullies, with terraces on top and vaults opened in the faces, is the rule rather than the exception.
And the third is the one that changes how you look. A yaodong is not a poor house: it is a house that solves two problems at once, material and temperature, using only what is there. The ground the Yellow River carries away as sediment is the same ground in which thirty million people have lived sheltered.
