David Goyder

For almost a century, maps of West Africa showed a huge mountain range stretching across the region along 10°N. At the time atlases and school maps all showed the Kong range. Folk even said its peaks were snow-capped. But, a teensy catch: the mountains never existed. 1/3

The Prime Meridian isn't where the brass line says. Stand on the line at Greenwich and GPS reads 0°0′5.3″ W: about 102 m (335 ft) west of zero longitude. In classic British tradition, the GPS zero actually runs through a bin in nearby Greenwich Park. The reason why? When Sir George Biddell Airy set up the famous transit circle telescope at the Royal Observatory Greenwich in 1851, astronomers used a basin of mercury to establish a perfect, vertical baseline ("down") relative to the stars. However, because the Earth is an irregular shape with an uneven distribution of mass, heavy underground rock formations subtly pull gravitational plumb lines off-centre, hence the 102 m discrepancy.