Antarctica has a 'gravity hole' where sea levels are 420ft lower

Antarctica is home to many mysteries, but one of the strangest is an enormous 'gravity hole' deep beneath the ice.

Gravity might feel stable and constant wherever we go, but its strength actually varies over the Earth's surface.

Where the gravity is weaker, the ocean's surface can actually dip well below the average as water rushes towards regions with higher gravity.

Scientists have known for years that gravity is at its weakest in an area of Antarctica's Ross Sea, where sea levels dip 420 feet (130 m) below the surrounding water.

Now, a pair of researchers claim they finally know why.

The vast gravity hole, known as the Antarctic Geoid Low (AGL), is the product of incredibly slow rock movements, according to the experts. 

Starting 70 million years ago – a time while dinosaurs still roamed the Earth – less–dense rock built up beneath the frozen continent and weakened the pull of gravity.

The gravity hole started small before rapidly growing in strength between 50 and 30 million years ago – creating the strange ocean dip that we see today.

When astronauts on the International Space Station look down on Earth, it might look like a smooth blue marble.

However, in reality, our planet is shaped a bit more like a 'bumpy potato'.

This lumpy surface is due to areas of non–uniform gravity, caused by the uneven distribution of material beneath the surface.

In regions where hot rock from within the Earth's mantle rises towards the surface, the lower density of rock means that gravity is weaker.

Since the 1940s, scientists have known that these gravitational anomalies cause deep depressions across large areas of the ocean.

However, figuring out how and why these gravitational anomalies form hundreds of miles beneath the surface is much harder.

To map how the Antarctic gravity hole formed, the researchers combined earthquake recordings from around the world with a computer model of the planet.

Co–author Dr Alessandro Forte, of the University of Florida, says: 'Imagine doing a CT scan of the whole Earth, but we don't have X–rays like we do in a medical office. We have earthquakes.

'Earthquake waves provide the "light" that illuminates the interior of the planet.'

By looking at how earthquakes travelled through different densities of rock, Dr Forte and his co–author constructed a map of the planet's inner workings.

Using a computer model, they then worked out where the gravity would be stronger and weaker based on the different types of rock.

Once their predictions matched the best current data from gravity–sensing satellites, they wound back the clock and watched as the gravity hole formed over aeons.

They discovered that the gravity formed slowly at first, before gaining strength in a period known as the Eocene Epoch – beginning 50 million years ago.

Interestingly, this coincided with enormous changes in Antarctica's climate, including the rapid advance of the continent's ice sheets.

Although it is not yet proven, the researchers suspect that there could be a connection between the formation of the gravity hole and the formation of Antarctica's glaciers.

Dr Forte says: 'If we can better understand how Earth's interior shapes gravity and sea levels, we gain insight into factors that may matter for the growth and stability of large ice sheets.'

In the future, the researchers want to hunt for a causal connection between the gravity hole and the ice sheets by creating new mathematical models of the climate.

Read More

Rare 'Ring of Fire' Solar Eclipse is about to appear in the sky - but only 100 PEOPLE will see it

Dr Forte says that this could answer one key question: 'How does our climate connect to what's going on inside our planet?'

The Antarctic Geoid Low is not the only massive gravity hole on Earth. 

Located in the Indian Ocean, the 'Indian Ocean Geoid Low' has such weak gravity that water levels dip 340 feet (103 m) below their surroundings.

In a recent study, a team of researchers in India claim the gravity hole was formed by plumes of low–density magma that rose up from Earth's mantle.

These plumes were generated by the remains of a sunken tectonic plate called Tethys, lost when India became part of Asia 50 million years ago.

The Earth is moving under our feet: Tectonic plates move through the mantle and produce Earthquakes as they scrape against each other

Tectonic plates are composed of Earth's crust and the uppermost portion of the mantle. 

Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.

Earthquakes typically occur at the boundaries of tectonic plates, where one plate dips below another, thrusts another upward, or where plate edges scrape alongside each other. 

Earthquakes rarely occur in the middle of plates, but they can happen when ancient faults or rifts far below the surface reactivate. 

These areas are relatively weak compared to the surrounding plate, and can easily slip and cause an earthquake.