The world’s longest cross-sea bridge has marked yet another milestone toward its completion. The monumental infrastructure is situated in South China’s Guangdong province, linking Hong Kong, Zhuhai and Macao. And as of April 10, a main tunnel has opened, linking Zhuhai and Macao.

Gongbei tunnel is a western part of the Hong Kong-Zhuhai-Macao Bridge. It’s only 2.7 km long, but what makes it special is that it passes right beneath a customs clearance point at the Gongbei border crossing linking Zhuhai with Macao — China’s busiest crossing point. And that made construction a lot more complicated — because the work could not disrupt customs officials, who check nearly 300,000 passengers a day.

To make that happen, a special pipe roofing was created for digging the tunnel. Engineers used dozens of curved steel pipes to form a huge outline of the tunnel. It was a very difficult process to install the pipes due to complicated geological conditions. Workers had to use a strong brine circulation to freeze the soil and mud outside the tunnel to make it solid enough for safety and to prevent water leaks. After the linkage, workers will continue further construction inside the tunnel. The bridge is projected to be fully operational by the end of this year.