Water covers around 70 per cent of the Earth's surface, but a huge amount also lies inside our planet.
Two new studies suggest that there may be vast quantities of water as far as 620 miles (1,000 kilometres) below the surface.
Without this huge store of water, the geodynamic activity that causes volcanoes - which are important for generating soil and sustaining life on the planet - would cease.
In the first study, researchers from Florida State University and the University of Edinburgh estimated that water exists far deeper in the Earth than previously thought, stored in a mineral called brucite.
Although the amount of water is unknown, researchers believe it could account for as much as 1.5 per cent of the weight of the planet - the same amount of water as all the world's oceans put together.
Mainak Mookherjee, who led the study, said: 'We didn't think water could be stored by hydrous minerals such as brucite at these depths.
'But now that we know it's there, we need to figure out how much water could be effectively stored inside it.'
In a parallel study, researchers from Northwestern University in Illinois suggest that this water is much deeper than any seen before, at a third of the way to the edge of Earth's core.
The researchers discovered a diamond, spat out 90 million years ago by a volcano near the São Luíz river in Juina, Brazil.
The diamond had an imperfection, containing minerals that became trapped during the diamond's formation.
When the researchers looked at it under the microscope, they saw evidence of the presence of hydroxyl ions - which normally come from water.
The nature of the imperfection suggests that it formed in the lower mantle.
Speaking to New Scientist, Steve Jacobsen, who led the study, said: 'This is the deepest evidence for water recycling on the planet.
'The big take-home message is that the water cycle on Earth is bigger than we ever thought, extending into the deep mantle.'
Water plays a key role in sustaining geological activity below the Earth's surface.
Dr Mookherjee told MailOnline: 'Water in the Earth's interior is crucial since it helps in mantle convection - a process by which solid rocks move from hotter to colder regions over geological time scales.
'If there was no water in the Earth's interior, mantle convection would be inefficient and would eventually cease.
'The surface expression of the mantle convection is plate motion (the process by which volcanoes are created).'
'Volcanoes play an important role in generating the Earth's crust on which we live.
'So if volcanic activity ceases then the crust formation will also stop and the planetary activities will eventually stop.'
Dr Jacobsen added: '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.'
Previously, researchers from the University of Alberta suggested the water locked up could account for as much as 1.5 per cent of the weight of the planet – the same amount as in all the world's oceans put together.
The researchers now plan to determine whether this is the case, by performing additional simulations to better understand the physical properties of brucite at that depth.
(DM)
