Scientists looking at water taken from deep within Earth's mantle say there is strong evidence it has been there since our planet's formation.
Writing in the journal Science, researchers say their findings suggest that ancient dust trapped at the center of the infant earth was already saturated with water.
That water still exists in tiny pockets inside a mineral known as olivine, which is embedded in volcanic rocks found in northern Canada and Iceland, the researchers report.
The pockets, containing just trace amounts of water, are tiny — less than 20 micrometers across.
"The measurements are extremely difficult to make," said Lydia Hallis, who participated in the research while at the University of Hawaii.
"Only in the past few years has the technology developed enough to measure such low concentrations of water inside such small amounts of material," said Hallis, now a research fellow at the University of Glasgow in Scotland.
Many scientists have believed that the water existing on Earth today came from water-rich comets or asteroids hitting the Earth after our planet had already formed.
However, clues to the origins of the water from Earth's deep mantle — found in the ratio of hydrogen to deuterium, known as "heavy hydrogen" — suggest otherwise, the researchers say.
That distinct ratio varies in the water present on different bodies in our solar system, which can help identify its source, the researchers explain.
"We found that the (mantle) water had very little deuterium, which strongly suggests that it was not carried to Earth after it had formed and cooled," Hallis said.
That suggests comets or asteroids may not have been the major source of our planet's water. Instead, water molecules were probably present in the disk of dust that surrounded our sun and out of which the planets later formed.
"Over time this water-rich dust was slowly drawn together to form our planet," Hallis suggested. "Even though a good deal of water would have been lost at the surface through evaporation in the heat of the formation process, enough survived to form the world's water."
The source of Earth's water has been the subject of a long-standing scientific debate.
While the study data suggest most of Earth's water has been here since its formation, Hallis acknowledges later additions may have come to Earth's surface and upper mantle reservoirs on comets or asteroids.
"What we can say is that the deep mantle water measured in our samples would have been isolated from this addition, as none of the later impacts would have been large enough to penetrate down to that level in the Earth," she concluded.