Greater Manchester spans 493 square miles (1,277 km2), which roughly covers the territory of the Greater Manchester Built-up Area, the second most populous urban area in the UK. It is landlocked and borders Cheshire (to the south-west and south), Derbyshire (to the south-east), West Yorkshire (to the north-east), Lancashire (to the north) and Merseyside (to the west). There is a mix of high-density urban areas, suburbs, semi-rural and rural locations in Greater Manchester, but land use is mostly urban—the product of concentric urbanisation and industrialisation which occurred mostly during the 19th century when the region flourished as the global centre of the cotton industry. It has a focused central business district, formed by Manchester city centre and the adjoining parts of Salford and Trafford, but Greater Manchester is also a polycentric county with ten metropolitan districts, each of which has at least one major town centre and outlying suburbs.
The introduction of textile manufacture initiated a process of rapid and unplanned urbanisation. A building boom began in Crompton during the mid-19th century, when suitable land in Oldham was becoming scarce. By the late-19th century Crompton had emerged as a densely populated mill town. At its spinning zenith, Shaw and Crompton was reported to have had more millionaires per capita than any other town in the world. Imports of foreign cotton goods began the decline in the region's textile industry during the mid-20th century; Shaw and Crompton's last mill closed in 1989. Today, Shaw and Crompton is a predominantly residential area of mixed affluence with a population of 21,721.
During the Second World War Turing worked at Bletchley Park, the UK's codebreaking centre, and was for a time head of Hut 8, the section responsible for German naval cryptanalysis. He devised a number of techniques for breaking German ciphers, including the method of the bombe, an electromechanical machine that could find settings for the Enigma machine.