Sydney's Super Tunnel

An extension is now underway to get people into the centre of Australia's biggest city, including a new tunnel crossing deep under Sydney Harbour.

With the construction of Australia's biggest public transport project in full swing, we head back under Sydney Harbour and witness the historic completion of the twin metro rail tunnels deep below the surface.

With 30 kilometres of tunnelling finished, it's time to start building railway stations, laying tracks and testing driverless trains. At Australia's busiest railway station, Central Station, the pedestrian subway known as Central Walks comes to life.

After almost 10 years of planning, building and testing, involving more than 60,000 people from around the world, Australia's largest transport infrastructure project nears completion.

The show kicks off with a dynamic taster of what the series has in store - namely the inside story of a crack team building a trailblazing, fully-automated metro system costing $50 billion, by far the biggest and most advanced new railway opening anywhere on the planet in 2019. A dramatic scene with our main characters dealing with a crisis gets the ball rolling.

We check in on our engineers encountering all manner of issues tunnelling through a minefield of existing infrastructure - skyscraper footings and basements, sewers, a graveyard, and existing rail tunnels.

Another super challenge looms, perhaps the greatest of the entire project - digging two tunnels beneath Sydney Harbour.In the early planning stages, the team actually considered dropping pre-fabricated tunnel sections down onto the floor of the harbour one by one, lowering them from barges, which was the way the road tunnel under the harbour was built.

We follow architect Ross de la Motte, seeing that his vision for the stations has finally come to light.He has gone to great lengths to design stations that benefit from natural light and airflow, minimising the need for additional power.Has he been successful?