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.