Since Edmund Hillary and Tenzing Norgay first reached the summit of Mount Everest in 1953, conquering the world's highest peak has been a goal of almost every mountaineer on the planet.

But this famous peak pales in comparison to two secret mountains, which are more than 100 times taller than Everest's 8,800-metre summit.

Reaching heights of around 620 miles (1,000km) these continent-sized 'islands' of rock dwarf anything else found on our planet.

However, confused adventurers can rest easy.

Scientists from Utrecht University have revealed that these gargantuan peaks are not found on our planet's surface.

Instead they are buried some 1,200 miles (2,000km) beneath our feet.

The researchers estimate that the mountains are at least half a billion years old but could date back to the formation of the planet four billion years ago.

Lead researcher Dr Arwen Deuss says: 'Nobody knows what they are, and whether they are only a temporary phenomenon, or if they have been sitting there for millions or perhaps even billions of years.'

The two monstrous structures sit on the boundary between Earth's core and the mantle, the semi-solid area beneath the crust, beneath Africa and the Pacific Ocean.

Around them is a 'graveyard' of sunken tectonic plates which have been pushed down from the surface in a process called subduction.

In a new study, researchers found that the islands are much hotter than the surrounding slabs of the Earth's crust and many millions of years older.

Scientists have known for decades that there are vast structures hidden deep within the Earth's mantle.

This is possible thanks to the way that the seismic shockwaves from earthquakes spread through the planet's interior.

When a powerful earthquake happens, it rings Earth like a bell, sending waves rippling from one side of the planet to the other.

But when these waves pass through something dense or hot, they are slowed down, weakened, or reflected altogether.

So, by listening carefully to the 'tone' that arrives on the other side of the planet, scientists are able to build up a picture of what lies beneath.

Over the years, studies have revealed that there are two enormous regions of the mantle where shockwaves dramatically slow down, dubbed the Large Low Seismic Velocity Provinces (LLSVPs).

Dr Deuss says: 'The waves slow down because the LLSVPs are hot, just like you can't run as fast in hot weather as you can when it's colder.'

When waves pass through a region that is much hotter, they need to expend a lot more energy to make their way through.

Co-author Dr Sujania Talavera-Soza says: 'Just like when the weather is hot outside and you go for a run, you don’t only slow down but you also get more tired than when it is cold outside.' 

That means you would expect the tone of a wave passing through the hot LLSVPs to be both out of tune and quieter than other areas, an effect scientists call damping.

However, when the researchers examined the data, they were surprised to find a very different picture.

'Against our expectations, we found little damping in the LLSVPs, which made the tones sound very loud there,' says Dr Talavera-Soza. 

'But we did find a lot of damping in the cold slab graveyard, where the tones sounded very soft.'

The pieces of rock from the crust cause lots of damping because they recrystallise into a tight structure as they sink down towards the core. 

This suggests that the mountains are made up of much larger grains than the surrounding slabs since these wouldn't absorb so much energy from passing seismic waves.

'Those mineral grains do not grow overnight, which can only mean one thing: LLSVPs are lots and lots older than the surrounding slab graveyards,' says Dr Talavera-Soza.

At the low end, the researchers estimate that these underground mountains are at least half a billion years old.

But they could be much older, potentially even dating back to the formation of the Earth itself.

This goes against the traditional idea that the mantle is in a constant state of movement.

Although the mantle is not actually liquid, it does move like a liquid over extremely long time frames.

Previously, it had been thought that the mantle would, therefore, be 'well-mixed' by flowing currents.

But the fact that these structures are billions of years old shows that they haven't been moved or disrupted by mantle convection, meaning that the mantle is not well-mixed after all.

Recently, scientists have suggested that the LLSVPs might be the remnants of an ancient planet which crashed into Earth billions of years ago.

Some researchers claim that the moon was formed when a Mars-sized planet called Theia collided with Earth, knocking molten chunks of both planets into orbit.

Since the moon is much smaller than the suggested mass of Theia, this leaves the obvious question of where the rest of the planet has gone.

Researchers from the California Institute of Technology have suggested that the LLSVPs could be the remains of the Theia collision.

After running a series of simulations, the researchers found that a significant amount of 'Theian' material — around two per cent of Earth's mass — would have entered the lower mantle of the ancient planet Earth.

That would explain why these regions seem to be so much denser, hotter, and older than the surrounding slab graveyard.

About 4.45 billion years ago, 150 million years after the solar system formed, Earth was hit by a Mars-size object called Theia.

The collision created the moon, but debate has raged exactly what happened during this event - and a mystery has persisted on why the moon and Earth are so similar in their composition.

The impact of Theia with Earth was so violent, the resulting debris cloud mixed thoroughly before settling down and forming the moon.

 

This cloud would have been composed of some Earth material, explaining the similarity between Earth and the moon, and other material.

The colliding body is sometimes called Theia, after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

But one mystery has persisted, revealed by rocks the Apollo astronauts brought back from the moon - why are the moon and Earth so similar in their composition?

Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon.

Perhaps the impact created a huge cloud of debris that mixed thoroughly with the Earth and then later condensed to form the moon.

Or Theia could have, coincidentally, been isotopically similar to young Earth.

A third possibility is that the moon formed from Earthen materials, rather than from Theia, although this would have been a very unusual type of impact.