From a red brick house in Rodney Street, Dover Heights, scientists found the centre of our galaxy.
A former secret wartime laboratory and military headquarters that was ''camouflaged'' as a house has been listed by the National Trust, acknowledging its little-known role as the birthplace of radio astronomy.
The cliff-top structure formed the heart of Sydney's coastal defence system during World War II, serving as a central observation and command post.
After the war, scientists stayed to investigate a mysterious static that had plagued radio operators. They built an antenna by digging a giant hole in the earth, and used it to pinpoint the Milky Way's centre.
The building at the site, which has since been cement rendered and painted, was built with decorative, art deco touches to hide its real purpose, said a National Trust industrial heritage expert, Tony Brassil.
''They have laid everything out so - especially from a distance - it looks less like a military building,'' he said.
The property, built in 1938, has since been converted to a home.
During the war, CSIRO scientists working with the military to test new radar technology at the site noticed a strange radio interference, which was attributed to enemy action.
At the end of the war, they investigated and found it was coming from noise bursts linked to sun spots and flares.
An operations scientist at the CSIRO's Parkes Observatory, John Sarkissian, said by turning their radar instruments to the stars, the scientists could study the universe ''in much greater detail''.
''Until then, astronomy was just what you saw through a telescope with your own eyes … radio astronomy opened up our knowledge of the universe to a different range of phenomena,'' he said.
In 1951, the scientists John Bolton, Gordon Stanley and Bruce Slee decided they needed a bigger telescope. With little government funding for radio physics, they spent their lunchtimes digging a 22-metre wide, dish-shaped hole. They lined it with steel strips from old packing cases and began surveying the Milky Way, said Dr Slee, the only one of the three still surviving.
''We did it on our own initiative. We thought it wouldn't be approved. We got our first results from it and got more support then,'' he said.
The scientists identified the centre of the Milky Way, a point later designated by astronomers as the ''zero point'' of the galaxy's co-ordinates.
The facility closed in 1954 but is considered a forerunner of the Parkes telescope.
The ''hole in the ground'' is now a sports field.