Why early European explorers believed Australia hid a massive, mysterious inland sea

History · Published 3 October 2026 Swipe more like this →

In short: For decades, European explorers were baffled by Australia's geography. Because major rivers flowed westward but failed to reach the coast, many concluded the continent must contain a vast, hidden inland sea. This theory drove explorers to endure extreme hardship, eventually leading to the discovery of the Murray-Darling river system.

The Great Dividing Range, a formidable mountain barrier shadowing the east coast, long thwarted westward expansion from Sydney. It was not until 1813 that Gregory Blaxland, accompanied by William Lawson, William Wentworth, and four servants, successfully crossed the Blue Mountains. This breakthrough opened the interior to colonial settlement and sparked a series of inland expeditions, including those led by George Evans and John Oxley. By 1815, Evans had reached the Lachlan Valley, and by 1817, Oxley was surveying the Lachlan River, though he famously concluded the interior was a marshy, uninhabitable wasteland.

As more rivers like the Darling, Macquarie, and Murrumbidgee were discovered flowing west, a compelling theory emerged: these waterways must feed into a great inland sea. This idea gained traction because Matthew Flinders, who had meticulously mapped the coast, found no major river deltas that could account for such significant inland drainage. The mystery was finally resolved in 1829–30 by Charles Sturt, who traced the Murrumbidgee and Murray rivers to their true outlet at Lake Alexandrina. This discovery proved the rivers drained into a tidal basin rather than a hidden sea, fundamentally altering the European understanding of Australian geography.

The quest for knowledge was not solely the domain of European explorers; Indigenous Australians played critical roles in these expeditions. Figures such as Jackey Jackey, who accompanied Kennedy, and Wylie, who traveled across the Nullarbor with Eyre, were essential to the success of these journeys. By the turn of the 20th century, most major geographical features had been mapped, though 20th-century figures like Ted Colson, who crossed the Simpson Desert in 1936, continued to push boundaries. Eventually, the era of ground-based exploration gave way to aerial surveys, with the Royal Australian Air Force mapping 1.25 million square miles of the continent between 1947 and 1952.

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