Analysis of an incredibly rare diamond suggests there may be as much water held in rocks below the Earth's surface as there is in all the oceans.
It contained a tiny amount of a mineral called ringwoodite which was found to have a significant amount of water within it.
The finding confirms scientific theories that there are huge volumes of water trapped between the upper and lower mantle around 410 to 660 kilometres beneath the Earth.
Prof Graham Pearson, who made the discovery in his role as Canada Excellence Research Chair in Arctic Resources, said: "This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area.
"That particular zone in the Earth, the transition zone, might have as much water as all the world's oceans put together."
The highly-prized diamonds that adorn jewellery form 150 to 200 kilometres below the Earth's surface but this one is an example of an 'ultradeep' specimen from the transition zone.
It was formed at pressures of an incredible 20,000 atmospheres and came to the surface through volcanic eruptions.
Although only one per cent of the ringwoodite is water, it is in fact a highly significant amount.
Prof Pearson told Nature News: "That may not sound like much but when you realise how much ringwoodite there is, the transition zone could hold as much water as all the Earth’s oceans put together."
The results of the analysis also help to shed light on other theories about how the Earth was formed.
It helps demonstrate, for instance, that hydrogen has always been an essential ingredient of our planet - and didn't have to be carried here by meteorites.