As it was, and as it is, an ocean on Earth (T. Fioreze)

If there is one thing our universe makes a lot of, it is water. This isn't an immediately obvious property based solely on the universal inventory of stuff. Hydrogen utterly dominates normal matter throughout the cosmos, and despite some 13 billion years of stellar nuclear fusion only a small number of these primordial protons have actually been digested by stars. Of these, most have ended up in helium nuclei, the rest in a tiny, almost imperceptible trace of elemental perfume. Compared to the number of hydrogen nuclei in the present-day universe, just a few tenths of a percent of atomic nuclei are elements heavier than helium. The most abundant however is oxygen, clocking in at roughly one nucleus for every 1,500 hydrogen nuclei.

But things like planets, and us, are all about these wisps of nuclear pollutants. And in relative terms oxygen and hydrogen combine to make water in stunningly colossal quantities. Recent observations of the dust and gas rich disks of matter surrounding nascent stars - the material that will coagulate to make planets in these systems - have revealed just how prevalent water can be; thousands of Earth-oceans' worth lurking in frigid vapor in these places. In our own solar system we are constantly reminded of the presence of this brand of ancient water, both in the salty pores of some types of meteorites, and in the spectacular tails of cometary bodies as they release their volatile molecules to the warming embrace of solar radiation.

Here on Terra-firma, water has been, and still is, a critical and central ingredient of both surface chemistry and the deep processes in our crusty lithosphere and mantle, including in the lubrication of tectonic movements. However, individual water molecules come and go, being split back out into their constituent oxygen and hydrogen atoms, and recombining or being produced from atoms that were formerly part of entirely different molecular species. A hydrogen and an oxygen that once belonged to one particular water molecule might find themselves recycled countless times and dispersed like unwanted orphans to the farthest reaches of the planet. Our terrestrial experience of water is really one of continual change.

But a recent discovery suggests that there can be circumstances on a planet like Earth where the components of specific water molecules can be preserved en masse in the most unexpected places for billions of years. Writing in Nature Geoscience, Shaw et al. report on a study of the composition of volcanic glasses produced in underwater eruptions a mile down in the ocean off Papua New Guinea. By studying the isotopic composition of the hydrogen nuclei (e.g. the deuterium content) and that of boron nuclei they found that this material has the fingerprint of ancient oceanic crustal plates in which the original water was forced into the crystalline rock structure and then 'distilled' by the heat and pressure, eliminating heavier istotopes. But these plates have been driven down (subducted) into the deep mantle of the Earth (at depths of hundreds of kilometers) for 200 million to a billion years, only now returning to the surface as the planet carries on its ancient routine of geophysical cycling.

So, perhaps you can't make a cocktail out of billion-year-old seawater, but you can certainly make a very, very dry martini with the essence of water molecules that were once in an ocean that existed at the dawn of our multi-cellular ancestors. As transitory as a planetary environment can be, it is also surprisingly full of the echoes of the past.