Scientists have claimed that massive oceans are hidden up to 620 miles beneath the surface of the Earth.

These mysterious bodies of water could even hold lifeforms so unusual that some scientists would regard them as aliens.

Steve Jacobsen, a researcher at Northwestern University in Illinois, said a part of Earth’s core called the mantle could contain several oceans’ worth of water.

The deepest of the subterranean seas is believed to stretch about a third of the way down to Earth’s molten core.

“If it wasn’t down there, we would all be submerged,” Jacobsen told New Scientist.

Jacobsen found a diamond in Brazil that contains imperfections that could only be formed in the presence of water.

“This implies a bigger reservoir of water on the planet than previously thought,” he said.

The underground oceans are unlikely to resemble any lakes or seas found on the surface of the planet, with the water bound within rocks which soak up the liquid like a sponge.

This means it is unlikely to be home to any large organisms that are known to science.

However, tiny microbes might just be able to survive in the hidden oceans.

Some scientists believe there is a “biotic fringe” in Earth’s core, which Live Science described as a “boundary where current knowledge predicts that no living cells persist” because the temperature and pressure are too high.

But researchers are often astonished by the conditions that organisms can put up with.

One single-celled organism called Geogemma barossii was found thriving near the deep-sea hydrothermal vents off the northwest US coast — an environment which Harvard University described as “alien.”

Geogemma barossii was able to survive in temperatures of up to 248 degrees Fahrenheit — something many scientists believed was impossible.

If this “alien” can survive in such extreme conditions, what else might be lurking down there in the depths of our planet?

Hopefully the “aliens” are as cute as this little squid: