the Leyland Trucks factory, the Hitachi Newton Aycliffe train plant, the Humber, Lindsey and Stanlow oil refineries, the NEPIC cluster of chemical works based around Teesside, and the nuclear processing facilities at Springfields and Sellafield. Offshore oil and gas from North Sea and Irish Sea, and more recently offshore wind power, are significant components in Northern England's energy mix. Although deep-pit coal mining in the UK ended in 2015 with the closure of Kellingley Colliery, North Yorkshire, there are still several open-pit mines in the area. Shale gas is especially prevalent across Northern England, although plans to extract it through hydraulic fracturing ("fracking") have proven to be controversial. Retail and services Around 10% of the Northern England workforce is employed in retail. Of the Big Four supermarkets in the UK, two – Asda and Morrisons – are based in the North. Northern England was the birthplace of the modern cooperative movement, and the Manchester-based Co-operative Group has the highest revenue of any firm in the North West. The area is also home to many online retailers, with startups emerging around tech hubs in Northern cities. With urban regeneration, high-value service sector industries such as corporate services and financial services have taken root in Northern England, with major hubs around Leeds and Manchester. Call centres – attracted by low labour costs and a preference for Northern English accents among the public – have replaced heavy industry as major employers of unskilled workers, with more than 5% of workers in all Northern England regions working in one. High-tech and research Together, the N8 research universities have over 190,000 students and contribute more to the Northern economy in terms of GVA than agriculture, car manufacturing or media. Discoveries and inventions at these universities have resulted in spin-offs worth hundreds of millions to local economies: the discovery of graphene at the University of Manchester produced the National Graphene Institute and the Sir Henry Royce Institute for Advanced Materials, while robotics research at the University of Sheffield led to the development of the Advanced Manufacturing Park. Recent decades have seen the growth of high-tech companies based around Northern England's major cities. There are eleven high-tech firms worth over $1 billion based in the region, and digital industries support around 300,000 jobs. Game development, online retail, health technology and analytics are among the major high-tech sectors in the North. Leisure and tourism The expansion of the