Do "aliens" exist on Earth? In a way, experts think so, and they believe that these creatures can be found thriving in massive underground oceans hidden hundreds of miles beneath the Earth's surface.

According to a report from the New York Post, these massive oceans lie 620 miles beneath our planet's surface. New Scientist notes that according to researchers, our planet's mantle could, in fact, have oceans.

“If it wasn’t down there, we would all be submerged," said Steve Jacobsen at Northwestern University in Evanston, Illinois, who discovered a diamond in Brazil 1,000 kilometers, or one-third of the way to the Earth's core. The discovery of the diamond is vital as it reveals that the gem's imperfections could only be formed with the presence of water.

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

If oceans underneath the Earth's surface do exist, Jacobsen notes that they would not look like the oceans we currently see today. These oceans would be bounded by rocks that act like sponges, soaking up the liquid from the water forms.

The possibility of water in the mantle means that these hidden oceans could be home to other organisms and microbes unknown to present-day science.

Jacobsen also said that his discovery prompts new possibilities that water could have existed on Earth longer than previously thought.

“Water clearly has a role in plate tectonics, and we didn’t know before how deep these effects could reach. It has implications for the origin of water on the planet," he said. “Water mixes with ocean crust and gets subducted at convergent plate boundaries. Introducing water into the mantle promotes melting and weakens rock, likely helping out the motions of plates like grease.”