Chinese mainland's leading innovation-tech hub and one of the most successful special economic zones is expanding vertically and underground to deal with a booming economy and a growing scarcity of land. William Xu reports in Shenzhen.

It was a foggy morning before the Lunar New Year in early February, with Zhang Youhui kept in suspense before the Shenzhen municipal authorities gave the green light for his drones to take off again.

Zhang is a manager overseeing deliveries of food, products and other supplies by drones at a commercial complex in the southern Chinese mainland city's Longgang district. Working for Meituan — a technology-driven retail company in China — he heads an eight-member team that facilitates the operations of about 20 drones that transports food and drinks via the air to consumers.

They had been informed earlier they had to suspend operations due to poor visibility. Zhang used the pause to examine his unmanned air delivery fleet. Free from the hazards caused by the terrain or traffic congestion, the mini-aircraft can travel up to 3 kilometers within 15 minutes, even during peak hours — a speed that's difficult for vehicles to achieve on the roads.