Deep in Yemen’s most remote valley lies the city of Shibam. The city of 2,000 inhabitants hardly seems impressive. Just a handful of high-rise residential buildings, not so different from the Soviet-style blocks found across the Arab world.\r\n\r\nYet these buildings don’t date from the 20th century, or even the late 19th century. They were built almost five centuries ago, and have remained largely unchanged since.\r\nThe city is one of the few to be designated a UNESCO World Heritage site. Its 500 houses reach an astounding 100 feet in height, almost as high as Chicago’s first skyscrapers. Perhaps most remarkable of all is the fact that the city was built using nothing but mud.