Life on Earth depends on the existence of water, but despite it covering some 70 percent of the planet's surface, scientists have never really been sure where it all comes from.

While the popular view has been that icy asteroids or comets could have brought water to Earth billions of years ago in an epic collision that filled the planet's oceans, a new study shows Earth's inner regions had the necessary ingredients to make water all along, hidden deep underground.

According to new computer simulations, chemical reactions taking place in Earth's upper mantle – which lies directly under the planet's crust – could produce water from scratch under the right (read: extreme) conditions.

"This is one way water can form on Earth," physicist John Tse from the University of Saskatchewan in Canada told New Scientist.

"We show it's possible to have water forming in Earth's natural environment, rather than being of extraterrestrial origin."

The reason this is possible is because of quartz, a common form of silica (aka silicon dioxide), which is widely abundant in Earth's crust and mantle.

Quartz is actually very stable, but in the upper mantle – which extends from Earth's crust down to a depth of about 410 km (250 miles) – enough heat and pressure can cajole the material to chemically react.

Specifically, the simulations show that once you venture deep enough into the upper mantle to reach a temperature of 1,400 degrees Celsius (2,552 degrees Fahrenheit) and a pressure 20,000 times greater than atmospheric pressure, quartz reacts with liquid hydrogen to produce silicon hydride and… liquid water.

"As long as the supply of hydrogen can be sustained, one can speculate that water formed from this process could be a contributor to the origin of water during Earth's early accretion," Tse told Andy Coghlan at New Scientist.

While a team of scientists in Japan ran lab experiments on this process back in 2014, Tse's team came up with some surprising findings of their own.

When they simulated the reaction, the liquid water didn't form in the way the scientists expected – being produced within the quartz crystals, not on their surface, as had been anticipated.

"We set up a computer simulation very close to [the Japanese] experimental conditions and simulated the trajectory of the reaction," said Tse.

"The hydrogen fluid diffuses through the quartz layer, but ends up forming water not at the surface, but in the bulk of the mineral. We analysed the density and structure of the trapped water, and found that it is highly pressurised."

Because it can't escape from the quartz once formed, the confined water – which the team estimates is under as much as 200,000 atmospheres of pressure – could bring about violent, unexpected seismic activity underground.

"We observed the water to be at high pressure, which might lead to the possibility of induced earthquakes," said Tse.

"However, further research is needed to quantify the amount of released water needed for triggering deep earthquakes."

While the reaction-based earthquakes remain a hypothesis for now, the findings could provide a new lead for geologists looking to explain powerful tremors that don't fit the regular mould – and help us understand more about movement within the mantle.

Earthquakes taking place deep within the lithosphere – the layer of Earth comprising both the crust and the upper mantle – have puzzled geologists in the past, and the quartz reaction could help explain some of those tremors.

"The formation and release of overpressured water may be a significant trigger in the deep lithosphere for ultra-deep earthquakes, sometimes located well below the crust and in the more rigid parts of deep continental plates," says executive director of the British Geological Survey, John Ludden, who wasn't involved in the study.

But while the researchers claim that their findings "have implications for fundamental geoscience questions [on] the origin of water on Earth", until we know more about the volume of water these quartz reactions actually produce, it may well be a very small amount, says Ludden.

So that means, for now, the ice comets hypothesis remains the most likely explanation for life as we know it on Planet Earth – until future research can give us a clearer answer on this ongoing mystery.

"I think it's reasonable to assume that much of the water came in this way," Ludden said.

The findings are reported in Earth and Planetary Science Letters.