Zhengzhou is a major manufacturing epicentre of China with a network of vast industrial zones and tech parks, including the Zhengzhou High and New Technology Industries Development Zone, the Zhengzhou Economic and Technological Development Zone, and the Zhengzhou Airport Economic Zone (ZAEZ). The city's most famous manufacturing enterprise is perhaps Foxconn's massive 250,000-worker plant that's located in ZAEZ. This factory, which is aptly dubbed "Apple City" for the main brand it manufactures products for, is the birthplace of 80 per cent of all iPhones in the world, producing over 100 million each year. In addition to Foxconn, ZAEZ is home to 14 other smartphone producers, who together supply 12.5 per cent of the global stock, and is currently the world's largest single site for smartphone production. Also located in ZAEZ are large operations in IT, biomedicine, aviation, and e-commerce. Last year, these enterprises amounted for 40 billion yuan (HK$47.6 billion) in total fixed commercial real estate investment, and 34 billion yuan in added value from manufacturing, and in 2014 produced 37.9 billion yuan in imports and exports, which was 59 per cent of the value of the total trade for Henan province. The ZAEZ is also growing fast, with a GDP that has risen by nearly 50 per cent since 2010 and a bonded area that has risen to become China's second-ranked free trade zone. Zhengzhou is also an epicenter of automotive production in the mainland, with Yutong Bus, Shaolin Bus, Zhengzhou Nissan and Haima Motor all having a major presence. Yutong Bus, China's largest bus and coach manufacturer, operates out of the 1.12 million-square-metre Zhengzhou Yutong Industrial Park, where it produced 67,018 buses last year, and has the capacity to produce 340 each day. In 2015, the company made 41.8 billion yuan and shipped buses to over 130 countries. Zhengzhou Nissan, a Sino-Japanese joint venture, manufactures 240,000 SUVs and pickup trucks, while Haima Motor produces 300,000 cars in Zhengzhou each year. Caterpillar, the Peoria, Illinois-based construction equipment company, also owns a subsidiary in Zhengzhou. Called Caterpillar (Zhengzhou), it is a large-scale manufacturing enterprise that produces hydraulic roof support systems for underground mining operations. They mostly supply the domestic coal mining industry, which is the largest in the world and consists of 80 per cent underground mines. Zhengzhou is also China's important base for frozen food production - a multibillion-dollar industry - of which two-thirds of the country's total supply is produced here. Half of the 10 largest frozen food producers are based in Zhengzhou, including Sanquan Foods, which operates a 5,000-worker factory in the city that pumps out over 100,000 frozen dumplings per hour, or over 400 tonnes of the Chinese staple per day. Synear Group, which commands 30 per cent of China's frozen food market, was founded in Zhengzhou in 1997 and still uses the city as its primary base of operations. Some of Zhengzhou's other key industries consist of textiles, carbonated beverages, cigarettes, fertiliser and processed meats.