SYDNEY, Dec. 9 (Xinhua) -- The first railway tunnel built underneath Australia's Sydney Harbor has been completed on Monday, with state officials congratulating construction workers 40 meters below the seabed.
"This is an engineering feat of historic proportions for our great city which will forever change how we get around Sydney," New South Wales (NSW) Premier Gladys Berejiklian said.
"Today we have made history walking deep beneath Sydney Harbor for the first time, inside one of two metro railway tunnels to be built as part of this mega project."
As Australia's largest ever infrastructure project by far, the entire 66-km Sydney Metro line which takes four years to build will have 31 train stations directly linking the north and inner west of the city.
But in order to cross the world's largest natural harbor, engineers had to undergo an extensive amount of research to determine the exact level of sea bed and what was below it.
Using old aerial images, geophysical surveys, bore hole data and scans that measure echoes of the sea floor, experts were able to determine the best location for the two tunnels.
To carry out the process, a giant 90-ton cutting machine called Kathleen had to be barged across Sydney Harbor and reassembled at the digging site.
Now set to begin the second tunnel, the machine was named after Kathleen Butler -- a woman who played a vital role in the construction of the Sydney Harbor Bridge, as a technical adviser to engineer John Bradfield.
"It is incredible that Kathleen has already finished her first tunnel and we are able to walk through this crucial piece of infrastructure deep under the harbor," NSW Transport Minister Andrew Constance said.