In China, more and more cities are joining the competition to build a network of metro lines. Our reporter Ge Yunfei has been following the construction of a metro station in China's southern city Guanghzou for a year. Let's see what he finds out.

China is urbanizing faster than any other nation on earth. And one of the best places to measure its progress is underground. 30 meters beneath Guangzhou - the capital of southern China's Guangdong province - construction workers are building a new subway line. This far down, temperatures can reach 60 degrees Celsius, as workers soaked in sweat use boring machines to excavate a tunnel.

MENG TAO, VICE GENERAL MANAGER CHINA RAILWAY NO. 3 ENGINEERING GROUP "Now we're working days and nights without any stop. Even when it's raining, we'll cover the site so the workers could stay on the field. As soon as the rain stops, the work will continue."

Up on street level, they're building a new station called Tianhe Park. It's as big as ten football fields. When completed it'll be the largest metro station in Asia. According to Meng Tao, building the station required the same amount of concrete as the massive Three Gorges Dam. Station construction has been underway for more than three years. In June 2017, it still looked like this. One year later, it's 90% finished-just waiting for the finishing touches. To build this mega project so fast, the company spared no expense.

MENG TAO, VICE GENERAL MANAGER CHINA RAILWAY NO. 3 ENGINEERING GROUP "This is a flagship project for us. We're trying everything we can to keep up with the schedule without considering the cost like adding more and more workers and increasing the investments."

What drove Meng Tao's company to invest so much capital is the huge potential market. The city now has over nearly 400 kilometers of metro lines and plans to double it within five years. By the year of 2020, China's total investment in metro lines could be up to four trillion yuan, or over 600 billion US dollars.