In 2018, China opened a crossing linking Hong Kong, Zhuhai and Macao with bridges and an undersea tunnel; the design avoided blocking major shipping lanes and airport flight paths

Most bridges try to stay above water. This one does something else. Midway across the Pearl River, the road dips down into a tunnel under the seabed and comes up again on the other side. According to a paper by engineers Ming Lin and Wei Lin, published in the journal Engineering, the tunnel section was one of the most technically demanding parts of the entire project. If you’ve ever wondered how a road can disappear into the sea, this bridge is a good place to start.

A bridge that disappears into the sea

The Hong Kong-Zhuhai-Macao Bridge, or HZMB, opened to traffic in October 2018. It connects three cities in the Pearl River Delta with an unbroken road. The full crossing is some 55 kilometers long and links Hong Kong, Zhuhai and Macao by road for the first time. The majority of the distance is an overpass. But about halfway, cars leave the bridge deck and go into a 6.7-kilometer tunnel that goes under the seabed. For this project, two artificial islands were built. They mark the transition points where the road descends into the tunnel and reappears on the bridge on the other side. NASA’s Earth Observatory compared the completed crossing to 15 Golden Gate Bridges lined up end to end, giving an idea of just how massive this project really is.


Why engineers decided to go underwater

This wasn't a design choice for aesthetic reasons. The Pearl River Delta is one of the busiest shipping areas in the world, and a tall bridge across the entire channel would have blocked large vessels. It is also near the airport in Hong Kong. A study in Tunneling and Underground Space Technology explained that this stretch had to be kept clear of the flight paths of planes landing at or taking off from Hong Kong International Airport, which ruled out a taller bridge over that section. Going under water solved both problems at once. The same study also notes that the Pearl River Estuary is considered a sensitive environmental area, and regulators imposed tight restrictions on the amount of disturbance the tunnel could cause to the natural water flow of the estuary.

Building a tunnel that can survive the sea

Building a tunnel under the sea is not like building a normal road. Workers couldn't just dig underground. Instead, they built the tunnel on land in 33 giant concrete sections and one final closing section, then towed each section out to sea and carefully lowered it onto the seabed one after the other like enormous puzzle pieces being put together. The sections were then sealed with rubber joints to keep out water when connected. Lin and Lin's research notes the tunnel's roof is buried as deep as 22 meters below the seabed surface in some areas, well beneath the natural water depth that already gives oil tankers as large as 300,000 tons deadweight room to pass overhead. The extra burial depth is instead there to protect the tunnel structure itself from scour, anchor strikes, and any future dredging of the shipping channel.

A bridge for the next 120 years

The bridge was designed to last for 120 years, far beyond the typical lifespan of most large crossings. Meeting this design life was genuinely difficult, given the harsh marine environment in the Pearl River Estuary, which is subject to frequent typhoons and strong currents, the Tunneling and Underground Space Technology paper notes. Understandably, that kind of ambition isn’t cheap either. Government figures widely reported at the time of its opening put the total construction cost at roughly 126.9 billion yuan, or about $18 billion. Construction itself took just over eight years, from December 2009 to early 2018, and was delayed more than once along the way.

What this great bridge means today

Beyond the engineering, the bridge has quietly changed everyday travel in the region. A road trip from Hong Kong to Zhuhai that took about four and a half hours to go around the delta can now be made in less than an hour. That kind of time saved can make travel much easier for people who move between these cities for work, family, or school. The idea of a road that goes underwater is not entirely new. NASA’s Earth Observatory notes that the HZMB uses a similar underwater-crossing concept that was previously implemented at the Chesapeake Bay Bridge-Tunnel in the U.S. and the Øresund Bridge linking Denmark and Sweden. What sets it apart is the scale at which the HZMB carried out that idea. It’s easy to scroll past a headline about a bridge and move on, but this one might be worth a pause. It shows how sometimes the smartest solution to a traffic problem might not be a bigger structure. It might be something that knows exactly when to disappear under the water.