Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 18

Something like another ocean's worth of water (and, in some esti-
mates, perhaps as much as 25 oceans 13 ) lie far below our feet, far below
even the groundwater reserves into which we drill down to obtain
much of our fresh water. These are not oceans, alas, like the ones
complete with storms, geysers, ichthyosaurs, and plesiosaurs that
Jules Verne's intrepid explorers sailed across in that nineteenth-
century classic Journey to the Centre of the Earth . The Earth's inner oceans
are dissolved in the rock and magma of the Earth itself. A tiny fragment
of the mantle mineral ringwoodite (previously only known from
ultra-high-pressure laboratory experiments) was recently found within
a diamond in Brazil, having somehow survived its 500-kilometre
journey to the Earth's surface. 14 It contained about 1.5 per cent water,
lending weight to the idea that parts of the mantle represent planetary
water stores.
The problem of the Earth's water is considerably larger, therefore,
than that of simply explaining the surface oceans. It is a conundrum:
one that is sharpened by the explosive climax to the Earth's
construction.
The Late Catastrophe
There is a wild card here, which surely had an impact—quite
literally—on what kind of oceans we now possess, and on the kind of
oceans we might once have had, long ago. It seems almost certain
now that the Earth, in its early history (some time in the first few tens
of millions of years), collided with a Mars-sized planetoid, a body that
has been called Theia. The frightful collision splashed out incandes-
cent magma, which then coalesced in orbit around the Earth to form
the Moon.
Such a collision would have been an extraordinarily large, and late,
reprise of the kinds of collisions that had built up the Earth shortly
before. The collision with Theia effectively explains the dynamics of