In terms of religion, 53% of the population are Christian, 6% are Muslim, 0.6% are Hindu, 0.4% are Buddhist, 0.2% are Sikh, 0.1% are Jewish, 0.4% belong to another religion, 31% have no religion and 7% did not state their religion. The largest quinary group is 20- to 24-year-olds (9%) because of the large university student population. The population of Sheffield peaked in 1951 at 577,050, and has since declined steadily. However, the mid-2007 population estimate was 530,300, representing an increase of about 17,000 residents since 2001. Although a city, Sheffield is informally known as "the largest village in England", because of a combination of topographical isolation and demographic stability. It is relatively geographically isolated, being cut off from other places by a ring of hills. Local folklore insists that, like Rome, Sheffield was built "on seven hills". The land surrounding Sheffield was unsuitable for industrial use, and now includes several protected green belt areas. These topographical factors have served to restrict urban spread, resulting in a relatively stable population size and a low degree of mobility. After many years of decline, the Sheffield economy is going through a strong revival. The 2004 Barclays Bank Financial Planning study revealed that, in 2003, the Sheffield district of Hallam was the highest ranking area outside London for overall wealth, the proportion of people earning over £60,000 a year standing at almost 12%. A survey by Knight Frank revealed that Sheffield was the fastest-growing city outside London for office and residential space and rents during the second half of 2004. This can be seen in a surge of redevelopments, including the City Lofts Tower and accompanying St Paul's Place, Velocity Living and the Moor redevelopment, the forthcoming NRQ and the Winter Gardens, Peace Gardens, Millennium Galleries and many projects completed under the Sheffield One redevelopment agency. The Sheffield economy grew from £5.6 billion in 1997 (1997 GVA) to £9.2 billion in 2007 (2007 GVA). The "UK Cities Monitor 2008" placed Sheffield among the top ten "best cities to locate a business today", the city occupying third and fourth places respectively for best office location and best new call centre location. The same report places Sheffield in third place regarding "greenest reputation" and second in terms of the availability of financial incentives. Sheffield has an international reputation for metallurgy and steel-making. Playing a crucial role in the Industrial Revolution, the city became an industrial powerhouse in the 18th century, and was dubbed "Steel City".