New research on the geology of Zealandia is revealing how it formed – and why it sank.

In 1820, a Russian ship packed with sailors and, oddly, penguins – destined for the men's dinner – spotted a towering shore of ice on the horizon. This was the first ever sighting of the Fimbul Ice Shelf, and it marked the official discovery of a new continent: Antarctica. It also cemented the modern idea, upheld by most maps across the English-speaking world, that there are seven major landmasses.

Today, schoolchildren, explorers and politicians generally accept the neat division of the world's ground into these simple units, which include Europe, Asia, Africa, North America, South America, Australia, and Antarctica.

But in 2017, the story took an unexpected turn – the seven-continent model has been a mistake all along. Enter Zealandia, a long-lost land to the southeast of Australia, otherwise known as the planet's forgotten eighth continent. Scientists had long predicted the existence of this bonus southern landmass, but it remained missing for 375 years – largely because it’s almost entirely submerged under 1-2 km (0.6-1.2 miles) of water. Now they are beginning to unravel its secrets.

This month, an international team of researchers released the most detailed maps of Zealandia to date – incorporating all five million square kilometres (two million sq miles) of this underwater region and its geology. In the process, they have uncovered hints as to how this mysterious continent formed – and why it has been obscured beneath the waves for the last 25 million years.

An ancient beginning

Zealandia is thought to have formed around 83 million years ago, during the Late Cretaceous. However, its journey began up to 100 million years before that, when the supercontinent of Gondwana – which congealed much of today's land into one giant lump – started to break up. As it disintegrated, the world's smallest, thinnest and youngest continent struck out on its own, while the regions of Gondwana that had once laid directly to its north west and south west became Australia and Antarctica, respectively.

It's thought that all or part of Zealandia may have existed as an island for a while. But then around 25 million years ago, it was disappeared beneath the ocean. (Read more from BBC Future about the missing continent it took 375 years to find.)

The first real clues that New Zealand might represent just a tiny, visible portion of a vast incognito landmass came in 2002, when scientists used bathymetry – the study of the depth of bodies of water – to analyse the region. The ocean above what we now call Zealandia is considerably shallower than that surrounding it, suggesting that the area was not underlain by an oceanic tectonic plate – like most of the world's oceans – but a continental one.

The clincher came in 2017, when scientists put several lines of evidence together, including data about the kinds of rocks it contains and its relative thickness – oceanic plates tend to be thinner – to propose that this is indeed a new continent. This is not a mere continental fragment or microcontinent, as had previously been proposed, but the real deal, 95% of which is submerged underwater.

However, despite the excitement around the discovery of a new continent, and more than a decade of intensive research, many details of Zealandia's early formation have remained elusive. This is partly because of a strange event that occurred when it split from Gondwana.

A tricky unravelling

In 2019, an international team of scientists mapped the geology of South Zealandia. Their research revealed that at some point, Zealandia had been stretched – pulled apart by tectonic forces, thinning the continent compared to regular continental plates and creating ruptures that later became oceanic crust. In the process, it became twisted and this made reconstructing its history to work out its original form much more challenging.

The researchers' analysis of rocks from the lost continent revealed that the stretching happened in two stages. The first began around 89-101 million years ago, and led to a rip which became the Tasman Sea inbetween Australia and New Zealand. The second phase started 80-90 million years ago and led to Zealandia splitting off from West Antarctica and creating the Pacific Ocean. But there were still many mysteries left unexplained – and the other half of this continent was yet to be studied in detail.

A gradual sinking

For the latest study, another research group – involving many of the same geologists as before – charted North Zealandia.

This time, they analysed rocks that had been dredged up from the Fairway Ridge, a region of the South Pacific off the coast of Australia, which forms the northernmost tip of Zealandia. These ancient remnants, which have not had a dry day for 25 million years, included a mixture of igneous rocks – those formed by volcanic processes – and sedimentary ones made in shallow basins just off the coast of Zealandia.

By analysing their chemistry and the radioactive isotopes deep within, the scientists estimated the rocks' age and origins. The oldest were pebbles dating to the Early Cretaceous (around 130-110 million years old), followed by sandstone from the Late Cretaceous (around 95 million years old) and relatively young basalts from the Eocene (around 40 million years old).

The resulting maps of Zealandia transform it from a featureless mass into a place with many bands of distinctive geology running along its length from northwest to southeast. These fit together with the geology of West Antarctica like a jigsaw puzzle, confirming that this region and Zealandia once slotted together.

The next stage of the investigation looked at measurements of magnetic anomalies in the ocean floor around Zealandia. These variations in the strength of Earth's magnetic field form an invisible record of how tectonic plates have moved around over time. They have revealed more about the continent's ancient stretching, which continued for millions of years and even changed direction – leading to an ultra-thin continent that eventually sank.

Zealandia is finally starting to give up its secrets. But with the vast majority of this enigmatic landmass under kilometres of water, it's going to take a while to reveal them all.

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