The 124,000-Year-Old Forest That Now Grows Under the Sea

On the floor of Spencer Gulf, South Australia, a 124,000-year-old forest stands exactly where it grew — trunks upright, roots buried in ancient soil, now drowned 30 metres beneath the sea. No one knew it was there until a fisherman saw strange shapes on his sonar.

The Forest Under the Waves

In June 2023, a diver exploring the seabed off Port Noarlunga noticed dark mounds rising from the sand. They turned out to be stumps of kauri and she-oak, preserved in peat that formed when sea levels were far lower than today. Radiocarbon dating confirmed the trees died about 124,000 years ago, at the end of the last interglacial period.

At that time, the coastline lay kilometres seaward of its present position. Spencer Gulf was dry land, cut by rivers and covered with woodland. When the ice sheets melted and seas rose again, the forest was buried beneath sediment and seawater — sealed in an oxygen-poor environment that slowed decay.

A Time Capsule of a Warmer World

The submerged forest records a climate slightly warmer than our own. The last interglacial peaked with global temperatures 1–2°C above pre-industrial levels, and sea levels 6–9 metres higher than today. The trees that grew on that now-submerged plain tell us what vegetation looked like when the planet was last this warm.

Pollen grains trapped in the peat reveal a landscape of casuarina, eucalypt, and banksia — a dry sclerophyll woodland adapted to a climate with less rainfall than modern Adelaide receives. The mix of species has no exact modern analogue, suggesting the ecosystem was a transitional one, adjusting to the changing conditions of a warming world.

The Pulse of the Ice Ages

The drowned forest is not a one-off. Similar submerged landscapes have been found off Tasmania, Western Australia, and the NSW coast — each one a snapshot of a different glacial stage. Australia's continental shelf, now underwater, was dry land for most of the past two million years, exposed each time the ice sheets expanded.

The sea that hides these forests is the same sea that drowned them, and it will drown them again.

These drowned landscapes are the archives of the ice ages, written not in ice cores but in peat and pollen and buried wood. They record the rhythm of the planet's climate cycles — the slow breathing of the Earth as it shifts between glacial and interglacial states. The Spencer Gulf forest is one breath, held for 124,000 years.

What the Sea Will Take Back

The peat that preserved the stumps is now eroding. Currents and storms are stripping away the sediment blanket that kept oxygen out. Within decades, the exposed wood will be colonised by marine borers and will disintegrate. The forest that survived 124,000 years underwater will soon be gone.

But its message is already read. The drowned trees of Spencer Gulf confirm what ice cores and coral terraces have told us: sea level can rise faster than we assumed, and it has done so before. The forest stands as a warning, not a curiosity — a reminder of how quickly a coastline can redraw itself when the climate shifts.