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Stand in the middle of the Australian outback and the heat shimmers off red earth in every direction. There are no coastlines here, no waves, no salt in the air. Yet beneath your feet, pressed into ancient rock layers across an area stretching from Queensland to South Australia, lie the remains of fish, marine reptiles, and ammonites. The outback was once a sea floor. The story of how it got there, and what it left behind, is one of the great geological tales on the planet.
The Eromanga Sea
Around 110 million years ago, during the Cretaceous period, a shallow body of water known as the Eromanga Sea covered a vast portion of eastern and central Australia. At its greatest extent, this inland sea stretched across what is now Queensland, New South Wales, and South Australia, dividing the continent into separate landmasses. Australia, as the world knows it today, simply did not exist in recognisable form. Scientists describe the continent at that time as looking more like an archipelago, with land only along parts of the eastern and western margins.
The waters of the Eromanga Sea were cold in the south and warmer toward the north. Marine reptiles patrolled its depths, alongside sharks, turtles, and vast numbers of ammonites. Pterosaurs flew overhead, and along the southern margins, a crocodile ancestor called the koolasuchus crawled through icy lagoons edged by forests. However, these were not tropical waters, the southern Eromanga Sea was near freezing, and the shoreline may have been covered in ice.
What the Sea Left Behind
The Eromanga Sea retreated gradually over millions of years, driven by volcanic activity and the slow uplift of land along what would become Australia’s northeastern coast. By around 95 million years ago, the sea had withdrawn completely, replaced by river plains and wetlands that themselves dried out over vast stretches of geological time. But the sea did not disappear without leaving clear evidence of its presence!
The sediments that settled on the Eromanga seafloor became the foundation of the Great Artesian Basin, the largest aquifer water system on Earth, now spanning over 1.7 million square kilometres and providing the only freshwater source across much of inland Australia. That ancient sea water, filtered through millions of years of sandstone, essentially still sustains life in the outback today.
The sea also left behind opals. As the Eromanga Sea receded, groundwater carried silica through the old seafloor sediments, and over time that silica hardened into the gemstones now found at Coober Pedy, Lightning Ridge, White Cliffs, and Andamooka. The opal fields of Australia sit almost entirely along the former margins of the Eromanga Sea. When miners dig for opals, they sometimes turn up fossils of Cretaceous creatures preserved in those same stones, making the outback one of the few places in the world where you can find opalized bones of ancient marine reptiles!
Megafauna on the Old Sea Floor
Long after the sea had gone, the flat inland basin it left behind became a very different kind of world. By the time of the Pleistocene, that same landscape supported enormous animals unlike anything alive today. The Diprotodon, a wombat-like creature roughly the size of a rhinoceros and the largest marsupial that ever lived, roamed the river plains of the Lake Eyre Basin. Giant tree kangaroos, some weighing around 40 kg, inhabited the forests that once covered much of inland Australia before the climate steadily dried out. A giant goanna known as Megalania, and horned tortoises, also moved through this land.
Fossil sites in Queensland’s Flinders Ranges and the Kalamurina region of South Australia have produced remarkable specimens of many of these animals. The fossils tell the story of a greening inland that dried and contracted over hundreds of thousands of years, pushing megafauna toward coastal fringes and eventually to extinction. The outback was very much alive, and for a long time.
Dreaming Tracks Across an Ancient World
Aboriginal people have lived on and navigated the Australian interior for tens of thousands of years, and their knowledge of the landscape carries its own kind of deep time. Songlines, also called Dreaming tracks, are paths across land and sky that record the journeys of ancestral creator beings during the Dreamtime, the sacred era of creation in Aboriginal belief. These paths are preserved in songs, stories, dance, and art, and they function as a system of navigation across the continent, encoding the location of waterholes, seasonal food sources, and sacred sites.
The Dreaming tracks crisscross the same ancient terrain that was once an inland sea that continues to hold water beneath its surface in the Great Artesian Basin. In this sense, Aboriginal tradition carries a map of a living landscape far older than any geological survey. The songlines represent, as one description puts it, corridors of knowledge that connect land, people, and creation across timescales most people can barely imagine.
Visiting the Ancient Sea Floor
The outback rewards slow and curious travel. The places where the Eromanga Sea left its mark are scattered across huge distances, from the underground opal mining town of Coober Pedy in South Australia, to fossil sites in western Queensland, to the aquifer springs that still show up to the surface at scattered points across the inland. These places demand context to be fully appreciated: a red cliff is just red rock until you understand that its colour and composition were shaped by an ocean that vanished 95 million years ago!
For travellers who want to explore Australia’s deep past with knowledgeable guides who understand all of its layers, The Thoughtful Traveler offers carefully designed tours that bring this extraordinary continent into focus. There is a great deal worth discovering beneath that sun-baked surface.
Author Bio
Lawrence Cohen, founder of The Thoughtful Traveler, is a lifelong explorer who believes travel should be as enriching as it is inspiring. After decades of globe-trotting, he grew weary of checklist-style tourism and began creating immersive journeys that connect history, culture, and human stories.
A seasoned traveler and author, Lawrence has visited six continents and over 40+ countries. He is pursuing a Master’s in Classical Studies at Brandeis University, and serves as a trustee of the Albright Institute, a leading archeological organization.
Based in Newton, Massachusetts, Lawrence continues to craft journeys that blend luxury, learning and genuine discovery.