Every year, millions of visitors stand at the clifftop lookouts along Victoria's Great Ocean Road and gaze out at the Twelve Apostles. These towering limestone stacks, rising up to 70 metres above the Southern Ocean, are some of Australia's most recognisable landmarks.
Yet despite their fame, no-one has ever really understood how they came to be. Until now.
In new research published in the Australian Journal of Earth Sciences, my colleagues and I finally answer that question - and the story involves ancient seas, shifting tectonic plates, and a transformation that began millions of years ago.
A window into deep time
The limestone of the Apostles contains an incredible archive of millions of years of history, and in particular climate history. But it has received relatively little attention from scientists.
Each layer was laid down in shallow seas during the Miocene epoch. This period in Earth's history, marked by a transition from warm to cool climate, lasted roughly from 23 million years ago to 5 million years ago. Each change from one layer to another represents a change in the local conditions such as temperature, chemistry, or movement of the water.
My colleagues and I carefully mapped the cliffs and sea stacks using high-resolution digital imagery alongside traditional fieldwork and sampling, and analysed fossils of microscopic sea creatures called foraminifera trapped in the rock. I calculated that one of the stacks contains around 760 trillion of these fossils.
As a result, we were able to read the layers of rock like tree rings.
This work has given us the most precise dates yet for the Apostles' limestone. Our fossil analysis shows the oldest layers of limestone are about 14 million years old, and the youngest about 8.6 million.
Beneath the limestone, visible at beach level east of the Apostles, is an older layer of soft, dark material called the Gellibrand Marl. This was deposited at the bottom of deeper, warmer seas around 14 million to 15 million years ago.
On top of the marl, forming the bulk of the cliffs and stacks themselves, is the Port Campbell Limestone. This was deposited in shallower, cooler conditions over the following several million years.
From 14.1 million to 13.8 million years ago, our fossil record captures a moment when the global climate was warmer than today. The layers from that time represent a natural record of what higher temperatures and sea levels look like, preserved in extraordinary detail on the Victorian coast.
Tectonics, tilting, and thrust faults
So how did limestone formed underwater end up standing tens of metres above the sea? The answer lies in plate tectonics.
As Australia drifted northward after splitting from Antarctica, changing stresses in Earth's crust compressed the region in a roughly northwest-southeast direction.
Starting around 8.6 million years ago, this compression buckled and lifted the limestone out of the sea. It didn't push the layers up perfectly straight.
If you look closely at the cliffs today, you can see the horizontal layers are tilted by a few degrees. Small faults are also visible in the cliff faces - the scars of ancient earthquakes caused by that same tectonic squeezing.
The cliffs are brand new
Our most surprising finding: while the rock itself is millions of years old, the dramatic coastal scenery we see today is brand new in geological terms.
The actual sea stacks and cliffs only took their present form in the past few thousand years, after sea levels rose about 125 metres following the last ice age, roughly 20,000-23,000 years ago.
As the sea flooded back in, waves began attacking the exposed limestone, which had also been weakened by tectonic forces. The rock fractured and eroded, forming headlands, then arches, which eventually collapsed to leave isolated stacks standing in the surf.
This process is still happening today. There were only seven or eight sea stacks (due to disagreements about what to count) when the Twelve Apostles were given their name - with a bit of poetic licence - in the early 20th century.
One collapsed in 2005 and another crumbled in 2009 , leaving a generally agreed number of seven today. The relentless toll of the waves means further collapses are inevitable, so we must pursue further research while we can.
A crucial climate record
The most exciting part of this research is not just what we have found already, but what remains to be read in these cliffs. We are now working to reconstruct the fine detail of how climate, sea levels and ocean conditions changed across those millions of years of history.
At a moment when the world faces urgent questions about our climate, the Twelve Apostles offer us an extraordinary record of where it has been and where we might be heading.
Stephen Gallagher does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
