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Travel to the tropics today and you'll experience balmy annual mean temperatures of 30°C (85°F).

But travel there 2.4 billion years ago, and it's likely you would have been subjected to Arctic-like temperatures of -40°C (-40°F).

This so-called 'Snowball Earth' theory suggests that our planet was once entirely frozen over - and it could have implications for finding life on other frozen worlds like Europa and Enceladus.

A University of Cologne scientist led research proposing a new theory. It suggests temperatures at Earth's equator were -40°C (-40°F) 2.4 billion years ago (artist's illustration shown). The reasons why the whole planet was frozen are not understood. But it could have implications for finding life on frozen moons like Europa

The research, led by Dr Daniel Herwartz from the University of Cologne, was published in the Proceedings of the National Academy of Sciences.

It suggests that the entire Earth was subjected to a 'deep freeze', with the oceans turning into ice 1,000ft (300 metres) thick.

This means that, were life to survive at the time, it would likely have had to remain in the water deep underground, below this icy crust.

On other worlds in the solar system such as Europa, we see a similar icy crust - and are fairly confident that it, too, has a liquid ocean underground.

'These events are fascinating. We had times where we really had a completely frozen Earth,' Dr Herwartz told Discovery News.

'If you go now to tropical regions and you imagine thick glaciers and all the oceans frozen, it's crazy I think, but it appears to be that this has happened.'

This theory can also help explain a period in Earth's history known as the Great Oxygen Transition, 2.2 billion years ago.

Here, oxygen levels began to rapidly rise from 0.0001 per cent of present levels to the 21 per cent of the atmosphere we see today.

Rapid melting across the entirety of Earth could have given rise to these increased levels of oxygen in the atmosphere.

Evidence for this latest theory comes from oxygen isotopes studied in rocks in China and Russia.

The exact causes for why the entire Earth seemingly froze over are not clear, though. 

Speaking to MailOnline, Dr Herwatz said before 2.4 billion years ago, the climate of our Earth was 'normal' - similar to the climate we have today.

And he said a drop in CO2 levels was likely the cause of Earth turning into a snowball.

'When CO2 drops ice caps grow large,' he explained.

'Because ice is white, a lot of the incoming sunlight is reflected back to space and so large ice caps cause the earth to cool even further.

'Climate modelling suggests that at a certain critical point, the Earth freezes over completely.

'Then, so much sunlight is reflected back to space (causing such a strong cooling effect) that the Earth becomes a snowball.'

The research, led by Dr Daniel Herwartz from the University of Cologne, was published in the Proceedings of the National Academy of Sciences. It suggests that the entire Earth was subjected to a 'deep freeze', with the oceans turning into ice 1,000ft (300 metres) thick (stock image shown)

It may have been that increased glaciation at the poles produced an opposite 'greenhouse effect', with the added ice reflecting more of the sun's light and making the planet ever-more cool, allowing the ice to spread.

Volcanic eruptions - or even a supervolcano - could also have reduced the amount of light reaching Earth, allowing temperatures to drop.

It appears that volcanic activity was ultimately responsible for helping Earth breaking free of its icy grip, though, increasing global temperatures.

In fact, Dr Herwatz said the amount of melting probably caused Earth to go through a 'supergreenhouse period'. 

The 'Snowball Earth' theory is not set in stone, though - some have instead suggested it may have been more of a slushball.

In other words, parts of the planet may have been encased in ice, but open seawater would still have existed in some locations, such as near the equator.

In Earth's past the continents were much closer together, and by 1.1 billion years ago they had formed a supercontinent known as Rodinia (left). Some moons in the solar system such as Enceladus (right) are covered in ice today, but believed to have oceans beneath their surface

Whether such an event could happen again, though, does not look likely. 

Dr Herwatz noted that 2.4 million years ago, the sun was not as strong as it is today, allowing the process to take place.

But a second snowball-like even 600 to 700 million years ago, which wasn't quite as severe, occurred when the sun was almost as strong as it is today.

'So I don't know why it should not happen again,' he said.

'All you need is an extension of ice caps to a certain threshold level.

'[But] considering the man made input of CO2 into our atmosphere we are currently heading in another direction, hence a Snowball Earth climate is currently very unlikely.'