Stretching nearly 30 kilometers, the world's longest cross-sea high-speed railway bridge over Hangzhou Bay in eastern China reached a key milestone on Friday when its first main tower was topped out.

As part of China's efforts to expand its high-speed rail network, the Hangzhou Bay Cross-Sea Railway Bridge spans one of the world's strongest tidal bays, which is home to the famous Qiantang River tidal bore.

The main tower rises 200 meters above the sea that is the height of an 80-story building. Offshore construction is especially challenging.

"The tide here can change by almost nine meters in a single day. But this bridge is designed to meet the standards for a 350 kilometer-per-hour high-speed rail, with each pier having a service life of 100 years. This presents a huge challenge," said Jiang Benjun, project manager of Hangzhou Bay Cross-Sea Railway Bridge at China Railway Major Bridge Engineering Group Company.

Construction teams have built a stable offshore working platform, effectively creating a "land base" at sea. At the same time, smart engineering technologies are playing a critical role.

"This system allows us to fully sense real-time conditions. Everyone, from managers to on-site workers, can instantly adjust operations for safety. All systems are independently developed in China," explained Jiang.

Upon completion, the bridge will set five world records in cross-sea rail engineering.

"As of now, the total investment in the bridge has reached 11.2 billion yuan (about 1.61 billion U.S. dollars), with 75 percent of the work completed. The bridge is expected to be fully connected by September this year. The entire project is set to be operational by the end of 2027," said Qian Liping, head of the command department of Hangzhou Bay Railway Bridge Project at Shanghai Guotie Engineering and Construction Management Company.

Once completed, high-speed trains will cross Hangzhou Bay in just around 10 minutes and the bridge will significantly enhance connectivity and support high-quality, integrated development across the Yangtze River Delta, one of China's most dynamic economic regions.