The tunnel-boring machine (TBM) Haitian advanced past the 3,000-metre mark on the southbound mainline of the Qingdao Jiaozhou Bay Second Tunnel (also known as the Qingdao Second Submarine Tunnel) on December 15, operating 91 metres beneath the seabed. The project is funded by Qingdao Conson Group and built by China Railway 14th Bureau Group (CRCC14). The milestone was reached ahead of schedule, laying a solid foundation for achieving the project's year-end targets.

The tunnel is currently the world's largest and longest undersea road tunnel under construction. The Qingdao Jiaozhou Bay Second Tunnel spans 17.48 km in total, including a 9.95-km subsea section. On the Qingdao side, shield tunnelling covers 3,255 metres, with the Haitian TBM featuring an excavation diameter of 15.63 metres.

Comprising 2 mainline tunnels and a central service tunnel, the project is designed for a top speed of 80 km/h and will serve as a cross-bay link for both passenger and freight vehicles.

Qingdao is expected to enter a dual-tunnel era once the tunnel opens to traffic. The urban areas of Qingdao proper and Huangdao will be closely linked by the Jiaozhou Bay Tunnel, the Jiaozhou Bay Second Tunnel, the cross-bay bridge and the ring expressway from south to north. The expanded network will help ease cross-bay traffic pressure and advance the integrated development of Qingdao and Huangdao area around Jiaozhou Bay, with major strategic significance for building a global marine hub city and a gateway for opening up.