If you've ever wanted to walk The Tank Stream, Sydney's first water source, here's your chance.
The tour, available only twice a year, takes you through 60 metres of this state heritage-listed tunnel built by convicts and stonemasons.
The stream was a vital source of life for the First Fleeters and the First Australians before them.
Captain Arthur Phillip chose Circular Quay as the birthplace of the new colony in part because of the freshwater stream running into the harbour from a swamp at the western end of what is now Hyde Park.
It had supplied fresh water and fish to the original Gadigal People for tens of thousands of years and served as the main fresh water supply for the first 40 years of Sydney's European life.
But by the early 1800s its waters were so polluted the colonists had stopped drinking from them. In time it became a sewer, still emptying into the harbour.
As the city grew up around the stream, it was covered over with sandstone blocks.
Today, The Tank Stream lies underground, a stormwater channel managed by Sydney Water.
Admission on the tour is by ballot registration only. Winners of the ballot have the chance to buy up to two tickets ($40 each).
For more information and to enter the ballot, click HERE