According to a recent survey, the Great Wall of China is much longer than previously estimated.
(It was estimated to stretch for around 5,000 kilometres, but a two-year mapping project, using global positioning systems and infra-red technology, carried out by the Chinese State Administration of Cultural Heritage has measured it at 8,851.8 kilometres (5,488.1 miles). Imagine anyone being so stupid as to try and build a wall to keep foreigners out today. Oh hang on...)