These strange circular structures are the "earth houses" of the Hakka people in Yongding county, in the south-east province of Fujian, an architectural exercise in communal living "in the round". It is certainly high-density living, a fortified village within one building. The oldest surviving round house, Chengqi, which was built in 1709, is 240ft in diameter, and still houses 40 families - all surnamed Jiang - making a grand total of more than 300 residents. "Very warm in winter, cool in the summer," was the much-repeated response of the inhabitants.
Imagine a four-storey polo mint-shaped apartment block, with an almost windowless circular outer wall and all rooms opening onto an internal round courtyard. Each extended family gets a four-storey slice of the cake, all arranged identically: outdoor kitchen and washrooms on the ground floor (including an array of pigs, ducks and guinea pigs for dinner), rice storage on the second, and living quarters on the two top levels. "There are very few arguments between the households, because with this design all the family units are the same and it is very equal," said one of the Jiangs. But privacy is not a strong point, and electricity did not arrive at many until the 1980s.
Yongding is the heartland of the Hakka people, whose name means "guest family". The Hakkas migrated from central China in several stages, starting more than 1,000 years ago, and settled mainly in the south-east of the country. Their house design developed over the centuries as a defence against bandits, the walls built of local clay, sticky rice and brown sugar, reinforced with wood and metal. Some 360 round houses are still in use.
Ethnically the Hakka are Han Chinese, but they have their own language and culture - and an impressive cast of characters. Taiwan's President Lee Teng-hui, the founder of post-imperial China, Sun Yat-sen, and the late paramount leader, Deng Xiaoping, are all claimed by the Hakkas as members of their extended clan.
In Yongding's Aoyao village, home to 15 generations of President Lee's forebears before they left in the 19th century, sits the Lee ancestral hall. There are even photographs of the Taiwanese president himself, normally banned on the mainland. One villager admitted he was "a little bit proud" of the village's only claim to fame, even though "he did not ever come here and he also refuses reunification".
President Lee's grandparents were part of an exodus of Hakkas who moved on in the late Qing dynasty. No-one seems quite sure how many Hakkas there are now in total, scattered mainly across Fujian, Guangdong, Taiwan, Hong Kong and south-east Asia.
The Hakka people's sense of cultural identity may be strong, but in Yongding county lack of jobs has prompted a dramatic latter-day economic migration. In Hongkeng village, the Lin family's Zhencheng round house is home to 36 people, compared to 200 earlier this century. The whole village, with more than 2,000 people, is populated almost entirely by grandparents and children.
"Most young people in the village go out as migrant workers and leave their children behind," said 35-year-old Mr Lin, who was born in Zhencheng and is now employed as the security man. About 90 per cent of his generation has left. "No one under 40 is here," he said.
And with the younger generation moving out, no one is building round houses. Qiaofu house, built in the early 1960s, complete with a Communist Party red star above the entrance, is possibly the most recent. Its owner, Jiang Zhenlin, now 69, used money sent by his brother in Burma. "He really loves round houses," said Mr Jiang.
Sadly, his house may be the last. "Right now," lamented Mr Jiang, "you cannot get the long wooden beams needed. This kind of building may take longer to construct, but it will last for several hundred years. Modern buildings can be built in a year, but only last 50 to 80 years."