A butt joint weighing 6,000 metric tons was lowered into the Pearl River on Tuesday morning, bringing work on an underwater tunnel linked to the Hong Kong-Zhuhai-Macao Bridge a step closer to completion.

Wang Zhanggui, chief engineer at the National Marine Environmental Forecasting Center, said favorable weather conditions made Tuesday suitable for connecting the butt joint, which involved 160 welders.

The tunnel is the final component of the long-awaited Hong Kong-Zhuhai-Macao Bridge, which is scheduled to open to traffic this year.

The Y-shaped bridge, which is estimated to cost more than 10 billion yuan ($1.5 billion), is expected to play a significant role in the economic development of the area comprising Guangdong province, Hong Kong and Macao.

Construction started in 2009, and the bridge is part of China's planned national highway network, linking the eastern and western banks of the Pearl River.

The bridge includes a 6.7-kilometer underwater tunnel and a 22.9-km overwater bridge, making it the longest cross-sea bridge in the world.

The service life of the bridge is expected to reach more than 120 years.