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For most people, continents are Earth鈥檚 seven main large landmasses.

But geoscientists have a different take on this. They look at the type of rock a feature is made of, rather than how much of its surface is above sea level.

In the past few years, we鈥檝e seen an increase in the discovery of lost continents. Most of these have been plateaus or mountains made of聽聽hidden from our view, below sea level.

One example is聽, the world鈥檚 eighth continent that extends underwater from New Zealand.

Several smaller lost continents, called microcontinents, have also recently been discovered submerged in the聽听补苍诲听.

But why, with so much geographical knowledge at our fingertips, are we still discovering lost continents in the 21st century?

We may have found another

In August, we undertook a聽聽on the research vessel RV Investigator to explore a possible lost continent in a remote part of the Coral Sea. The area is home to a large underwater plateau off Queensland, called the聽, which represents a major gap in our knowledge of Australia鈥檚 geology.

On one hand, it could be a lost continent that broke away from Queensland about 60 million years ago. Or it could have formed as a result of a massive volcanic eruption taking place around the same time. We鈥檙e not sure, because nobody had recovered rocks from there before - until now.

We spent about two weeks collecting rocks from this feature, and recovered a wide variety of rock types from parts of the seafloor as deep as 4500m.

Most were formed through volcanic eruptions, but some show hints that continental rocks are hiding beneath. Lab work over the next couple of years will give us more certain answers.

Down to the details

There are many mountains and plateaus below sea level scattered across the oceans, and these have been mapped from space. They are the lighter blue areas you can see on Google Maps.

However, not all submerged features qualify as lost continents. Most are made of materials quite distinct from what we traditionally think of as continental rock, and are instead formed by massive outpourings of magma.

A good example is聽聽which, despite being roughly the size of New Zealand鈥檚 North Island, is not considered continental in geological terms. It鈥檚 made up mainly of volcanic rocks deposited over the past 18 million years, meaning it鈥檚 relatively young in geological terms.

The only foolproof way to tell the difference between massive submarine volcanoes and lost continents is to collect rock samples from the deep ocean.

Finding the right samples is challenging, to say the least. Much of the seafloor is covered in soft, gloopy sediment that obscures the solid rock beneath.

We use a sophisticated mapping system to search for steep slopes on the seafloor, that are more likely to be free of sediment. We then send a metal rock-collecting bucket to grab samples.

The more we explore and sample the depths of the oceans, the more likely we鈥檒l be to discover more lost continents.

The ultimate lost continent

Perhaps the best known example of a lost continent is聽. While the geology of New Zealand and New Caledonia have been known for some time, it鈥檚 only recently their common heritage as part of a much larger continent (which is 95% underwater) has been accepted.

This acceptance has been the culmination of years of painstaking research, and exploration of the geology of deep oceans through sample collection and geophysical surveys.

New discoveries continue to be made.

During a 2011 expedition, we discovered聽聽more than 1,000km west of Perth.

The granite lying in the middle of the deep ocean there looked similar to what you would find around Cape Leeuwin, in Western Australia.

Other lost continents

However, not all lost continents are found hidden beneath the oceans.

Some existed only in the geological past, millions to billions of years ago, and later collided with other continents as a result of plate tectonic motions.

Their only modern-day remnants are small slivers of rock, usually squished up in mountain chains such as the Himalayas. One example is聽, an ancient continent now embedded in the mountain ranges across Europe.

Due to the perpetual motion of tectonic plates, it鈥檚 the fate of all continents to ultimately reconnect with another, and form a supercontinent.

But the fascinating life and death cycle of continents is the topic of another story.

This story was first published on .

Declaration

Maria Seton receives funding from the Australian Research Council and has received ship time funding through Australia's Marine National Facility.

Joanne Whittaker receives funding from the Australian Research Council, the Australian Antarctic Division, and ship time through Australia's Marine National Facility.

Simon Williams is affiliated with the University of Sydney and Northwest University, Xi'an. He receives funding from the Australian Research Council and the Natural Science Foundation of China, and ship time through Australia's Marine National Facility.