The underwater tunnel section of the Hong Kong-Zhuhai-Macao Bridge was completed in July, marking a milestone for the mega-project.

But why did the tunnel needed to be built underwater?  Chinese engineers answer to the challenge of what many people thought was a "mission impossible."

Four thousand vessels pass through the busiest waters stretching nearly 7 kilometers in the Lingding Channel. This means it is simply not feasible to build a bridge with guaranteed clearance for every ship. So engineers decided to go low.

Rather than a channel-spanning bridge, builders opted to try shielding - an advanced tunneling technique that would make their 18-month deadline seem more possible to meet. But simulated results showed that in order for the shielding to be successful, the two artificial islands at both ends of the tunnel needed to be much larger than planned, which would lead to irreversible damage to the area's natural water flows.

This is when Plan C, to sink concrete tubes onto the seabed and dock them underwater, came into action.

Each tube is 180 meters long and weighs over 76 thousand tons, almost the size of an aircraft carrier and far too large for a common factory. So builders decided to break each tube into eight smaller units, and build each unit separately. After that, the units are welded together within a 3-centimeter margin of error.

The total length of the tunnel is 6.7 kilometers. Altogether, it is made of 33 tubes, sunk in over 40 meters of water, to form the longest and deepest immersed tunnel in the world.

On July 7, 2017, this unprecedented undersea project was fully completed.