An underwater tunnel connecting the north side to the south side of Shantou Harbor in Shantou, opened to traffic Wednesday, significantly cutting travel times in the Guangdong province city.

Construction on the 6.68-kilometer tunnel began in 2015 and cost 5.7 billion yuan ($790 million).

Engineers on the project had to overcome significant challenges facing the tunnel's construction, such as it passing through various compositions of rock and being located in an area that has historically recorded strong earthquakes.

"Technological breakthroughs have been made during the construction of the sea tunnel," said Shi Qu, an engineering manager at Shantou Su'ai Tunnel Construction Investment and Development, the tunnel's builder.

To ensure the circulation of fresh air in the tunnel, wind towers on its northern and southern sides pump air and discharge exhaust gases in equal measure.

"The wind towers, along with 24 jet fans installed inside the tunnel, create a breathing system," said Shi.

Before the tunnel's opening, drivers had to use one of two cross-sea bridges to get from one side of the city to the other across the harbor, which took on average 40 minutes. The tunnel shortens the journey to 10 minutes.

"The most exciting thing is that the time it takes to cross Shantou Harbor has been greatly shortened," said Pan Yanyan, a manager at a local bank.

In recent years, Haojiang district on the southern edge of the harbor has become a popular leisure destination during holidays.

"The opening of the sea tunnel will help boost urban development on the southern side of the harbor," Pan said.