Before Mount Ainslie and Mount Majura existed, the land that became Canberra was the floor of an ocean that ended many kilometres away from today's national capital.
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The rocks beneath Parliament House tell this story like a book - except it's one that reaches an abrupt halt.
There's a page missing, so to speak.
Geologist Wolf Mayer uses this analogy to explain how a wall of rock deep inside Capital Hill, unremarkable to the untrained eye, is actually a portal to a land that Canberrans wouldn't recognise.
Dr Mayer can translate the face of the rock: A stretch of Black Mountain sandstone shows that, at some point about 430 million years ago, a clash of tectonic plates forced the land up above the water.
For millions of years it was eroded by the forces of weather.
What happened next isn't so clear. The story picks up again at a different time, when Canberra was back underwater and home to coral and other marine life.
The missing geological page is called an "uncomformity". The word describes when two rock masses, from different time periods, are joined together.
As Dr Mayer explains, an uncomformity is a rare time gap for anyone reading the rocks for clues about the land's story. Under Parliament House, the rocks are missing a geological record of about 10 million years.
"It's just been wiped, it's gone," he says. So Dr Mayer reads on, moving further up the rock face where there are layers of a different stone.
Geologists have used these to figure out what happened during the gap.
The next chapter is the "Canberra formation", telling of a land re-invaded by the sea, albeit a shallower one.
Mount Ainslie, the product of volcanic activity, was still many years down the track.
"In a sense you might think of an unconformity as a negative feature as it erases so much of the geological history," Dr Mayer says.
"In fact it does the opposite. Some of the features below and above, we can actually use to reconstruct what happened in that gap.
"In that sense, an uncomformity is a very positive thing to have."
Before 1981, anyone able to translate these time gaps only needed to walk up Capital Hill to see them in exposed rocks.
When it was chosen to host Australia's permanent Parliament House, authorities rejected geologists' attempts to have the rocks preserved.
Dr Mayer thought that part of Canberra's geological history was flattened and concreted over until about 1995, when a visitor's services guide said there were rocks under the building. He was taken underground to look.
"I couldn't believe my eyes when I saw these rocks stretching out across the basement here," he says.
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Parliament House's builders left behind some of the ancient clue into Canberra's past.
"They themselves didn't anticipate that the rocks would be left. Once they dug deep enough, they left the rest," Dr Mayer says.
He's given tours of the rocks since, relating the story they tell. His next tours, on August 10-11, coincide with Science Week. Tickets are limited and selling fast.
The fascination never ceases for Dr Mayer, who sometimes takes international visitors to Parliament House to see the rocks.
"It's a very important feature to help us understand the very ancient history of the Canberra region."