Chinese engineers had never built an immersed tunnel on the open sea, let alone one that, upon completion, would be the world's longest, at a total length of 6.7 kilometers, and one of the deepest, at almost 45 meters, under China's Pearl River Delta.

The idea of directly linking the economic powerhouses of Hong Kong and Macau—shortening the three-hour drive to about a half hour—had floated around for almost 20 years. But it's no wonder the goal has not been met until now: The approximately 50-km-long new highway system has to squeeze past the Hong Kong airport, including air-space restrictions; traverse the aquatic version of a busy interstate highway; and lie in variable layers of soils, ranging from mucky to clay-like to sandy.