Sydney Metro project director Rodd Staples equates the construction of the new stations to "keyhole surgery" compared with past projects. Credit:Edwina Pickles Historians have long known that the area around Sydney Harbour was a "drowned" valley - a vegetated area that became inundated along with the melting of continental ice caps about 20,000 years ago. "We always knew that there was some sort of watercourse there, but this [survey] has helped us define it," said Rodd Staples, the senior bureaucrat who is overseeing what the transport department is calling the metro rail project. The survey shows the Hawkesbury sandstone that lies beneath the sediment is deeper than previously known - up to 15 metres lower than thought in areas the Harbour Bridge. It also shows the path of the original river that ran through the modern-day harbour.

Shells in the sediment 38 metres below the surface of Sydney Harbour were undisturbed for 20,000 years. That has been revealed to be a relatively unimpressive watercourse, of the scale of the Woronora or Lane Cove rivers. The harbour has, of course, been crossed twice before. Though an undersea crossing was discussed at the time John Bradfield was engineering the bridge, the idea was thought fanciful. A geotechnical map for a new rail project shows the contours of rock beneath the surface of Sydney Harbour. The existing Sydney Harbour road tunnel, completed in the early 1990s, only so much as scratched the surface: its pieces were laid end-to-end at the bottom of the harbour and then dredged.

"No one has ever dug this deep," Mr Staples said. "We've found that the rock was much deeper than we ever expected." The rock was much deeper than we ever expected. Rodd Staples Knowing where this sandstone lies is important for engineers, who want to choose a location for the rail line that isn't too deep for passengers to get to but is also stable enough to be drilled. Some layers 50 metres below the surface show signs of charcoal and suggest the valley was struck by bushfires. The excavation has also unearthed snails, worm burrows and shells not disturbed for millennia. That material is being sent away for carbon dating and is expected to be the subject of future research. The new project is set to begin in 2017 and passengers are expected to be able to travel along it by 2024.

The new line is planned to connect with the north-west rail link to Rouse Hill, currently scheduled for completion in 2019. The state government believes the projects will be key to reviving Sydney's overstressed rail network. "Our current system can move 24,000 people an hour ... with this new metro system we can carry more than 40,000 people per hour," Mr Staples said. The government will now go to the international market seeking boring machines.