Underground cathedrals hidden beneath Sydney to form new metro line

It's like a step into Sydney's new underworld.

Buried about 25 metres beneath Hunter and Elizabeth streets in the CBD lie two giant caverns, and connecting tunnels, excavated for Sydney's multibillion-dollar metro rail line.

On the streets and high-rise towers above, thousands of people go about their days oblivious to the engineering feat beneath them to create a new station.

One of the giant station caverns for the multibillion-dollar metro line under construction beneath central Sydney.Credit: Matt O'Sullivan

The caverns for the Martin Place metro station illustrate the scale of Australia's largest transport project which, by 2024, will carry tens of thousands of passengers every hour on driverless, single-deck trains.

Each cavern is about 17 metres high and 14m wide, and will eventually feature 160m-long platforms at which trains will pull up at every four minutes.

Similar construction is under way for the three other underground stations in the CBD at Central Station, Pitt Street near Town Hall, and Barangaroo.

Transport Minister Andrew Constance, Premier Gladys Berejiklian and Transport for NSW secretary Rodd Staples inspect the station site.Credit: Joel Carrett

The excavation of the caverns at the stations need to be completed before two giant boring machines, worming their way from the city's south, reach the CBD later this year.

Construction of the metro line beneath the CBD is in contrast to the troubled $2.1 billion light rail line, which has faced long construction delays along George Street in plain sight of Sydneysiders.

On the eve of the caretaker period before the state election, Premier Gladys Berejiklian was eager to spruik her government's signature transport project as evidence of the Coalition's ability to deliver.

"Not only are we providing turn-up-and-go, world-class rail services, but we're also providing amazing pedestrian access underground," she said of the new station.

One of the caverns for the Martin Place metro station. Credit: Joel Carrett

"Many of us walk the streets above ground in the CBD and have no idea until today the absolute magnitude of what's going on."

The first stage of the $20 billion-plus new line is due to open in May. The Sydney Metro North West stretches from Rouse Hill to Chatswood, where it will connect to the second stage to the CBD, Sydenham in the south and Bankstown in the west.

Transport for NSW secretary Rodd Staples said one of the challenges in constructing the line beneath the CBD would be navigating the tunnelling machines around building basements.

"There is a lot of monitoring equipment within the tunnels and around the buildings to make sure we know about any movement," he said. "We do expect small amounts of movement but we want to make sure that movement is within the tolerances we expect."

However, the most challenging part of the tunnelling  will be under Sydney Harbour because of the clay, sediment and rock the engineers face in building the mega project.