Episode 2: Secret behind China's new world-largest tunnel: Develop economy or save Three Gorges Dam?

What was China willing to lose to protect the so-called title of the world's biggest dam?

Here is a recap of episode one.

Since its construction began in 1994, the Three Gorges Dam, China's largest hydroelectric dam and also the world-largest dam, has dramatically drawn worldwide attention. So far, the dam has remained the world's most controversial construction.

Twenty-eight years on, and Beijing recently boasted about another world first. China began construction of the new tunnel project named Yinjiangbuhan on July 7 designed to send water from the Three Gorges Dam to Beijing.

The tunnel will run a total of 870 miles long, connecting the Three Gorges Dam to the Han River and then on to Beijing, and parts will go as deep as 3,300 feet underground. It will be much longer than the Päijänne tunnel in Finland, which is currently the longest continuous rock tunnel in the world.