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Iceland may not just be an island 鈥� it could be the only exposed part of an entire continent, dubbed Icelandia, that is mostly submerged beneath the Atlantic Ocean.

鈥淭here is a hidden continent right there under the sea,鈥� says at Durham University in the UK. She and her colleagues have published the idea in a of a new book, .

Iceland lies on the Mid-Atlantic ridge, where two of the tectonic plates that make up Earth鈥檚 surface are slowly moving apart. Hot magma from inside听 the planet wells up along the ridge, before cooling and solidifying into rock, forming more seabed.

Earth鈥檚 outer layer, the crust, is typically thinner under the oceans than it is under continents 鈥� but Foulger says . Geologists have assumed that it is made of oceanic crust that has accumulated over millions of years, but it is hard to explain how so much could have formed, according to Foulger.

鈥淔or the ocean floor, the crust is typically 6 to 7 kilometres thick,鈥� she says. 鈥淏ut [the crust beneath] Iceland is 40 kilometres thick.鈥�

In general, geologists argue that the thickness of the crust can be explained by the presence of a so-called geological hotspot 鈥� an unusually hot region in the mantle that leads to greater volcanic activity.

But Foulger and her team have an alternative explanation. They argue instead that Iceland is made of continental crust and so are large areas of the surrounding seabed. This explains its odd features, says Foulger. 鈥淓verything fits. Why didn鈥檛 we see that before?鈥�

This hidden continent of Icelandia, if it exists, has a surface area of 600,000 square kilometres. A second region that is north-west of Scotland and includes the Faroe Islands might also be part of it 鈥� which would give it a surface area of 1 million square kilometres.

Icelandia would be a relic of a time millions of years ago, when the continents that are now on opposite sides of the Atlantic Ocean were joined in a single supercontinent called Pangaea.

The continents have since been pulled apart by the motion of the plates. However, Foulger thinks Icelandia is one chunk of Pangaean continental crust that was unusually tough and has been 鈥渟tretched and stretched and stretched鈥� without breaking, and now sits underneath Iceland.

鈥淚t implies Pangaea hasn鈥檛 yet broken up,鈥� says Foulger. Her team has funding to collect very resilient crystals called zircons from Iceland. If the deeper rocks really are continental crust from Pangaea, the zircons should help support the idea.

Continental crust is typically billions of years old, whereas oceanic crust 鈥� which is constantly created and destroyed 鈥� is usually no more than a few hundred million years old. So if the zircons beneath Iceland are older than this, they would support the team鈥檚 idea.

Iceland probably isn鈥檛 unique, says Foulger. 鈥淪omething similar could be happening at many more places.鈥� In recent years, geologists have found evidence that the islands of New Zealand are the exposed parts of a much larger continent called Zealandia.

Foulger says there are many other regions of seabed that might be drowned fragments of continent 鈥� for example, around the Kerguelen Islands in the southern Indian Ocean.

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