Lots of interesting stories get trapped inside minerals. Zircons can host clocks that can tell us the ages of the oldest pieces of our Solar System. Diamonds hold onto little bits of the Earth’s mantle that they captured before being blasted upward through the crust. Sometimes crystals can even contain small amounts of fluid locked away since the time they solidified.

The authors of a new study went looking for some of that ancient water in a slightly different place—fractures in extremely old rocks. What they found in that water recorded an impressive amount of history.

Deep in a Canadian mine, the shafts pass through some rocks that have seen a lot. About 2.7 billion years ago, sediment was formed from broken down bits of even older rocks that are now lost to us. The layers of sediment are frequently interrupted by thick layers of lava flows containing the copper and zinc that would one day entice humans to burrow 2.4 kilometers down into the darkness. The sediment was eventually buried deeply enough that, after solidifying into rock, it was metamorphosed into even tougher stuff. During that time, it was also bathed in super-hot hydrothermal fluids.

It appears that some of that fluid has been trapped in the rocks ever since. But how can we know this? How can a vial of water tell you what it’s been up to? Well, you have to ask the chemical tenants within it.

The most interesting of these residents are the non-reactive noble gases dissolved in the water. The ratio of the isotopes of helium present show that almost all of the helium came from the radioactive decay of other elements—it takes time for that much to accumulate. That story is corroborated by the argon isotopes, too. The neon looks like a mixture of neon from the atmosphere with a healthy dose of the products of radioactive decay, similar to how fluids locked in very old minerals look.

But it’s the xenon that really starts coughing up information. There were nine different isotopes of xenon in the samples, and they were able to answer a range of questions. First, they bear the mark of fluids that interacted with magma—a remnant from the hydrothermal action. And the fingerprint once again looks like something that has been isolated from the atmosphere for a very long time. In fact, it preserves the isotopic composition of the ancient atmosphere, which has slowly evolved over the ages.

Based on the gasses within it, the water looks like it has been isolated, but for how long? According to our understanding of how atmospheric xenon changed over time, the signature in the samples looks like air from 1.5 to 2.6 billion years ago. Based on the amount of radioactive uranium, thorium, and potassium, you can also ballpark how long it would take for the daughter isotopes (the breakdown products of those radioactive elements) of helium, neon, argon, and xenon to accumulate. That estimate comes out to roughly 1.1 to 1.7 billion years, depending on which isotope you look at.

So the evidence shows that this water has been sitting around in these rocks for at least a billion years, and possibly up to 2.6 billion years. (Cue the rush to bottle and sell it as some sort of fountain of agelessness…)

If the age is not enough for you, there may be an even more interesting chapter to follow. Along with all these noble gases are some that are more prone to engaging in chemical reactions. In particular, there’s plenty of H 2 and methane—compounds that can make microbial life possible. If there were, say, single-celled archaea living in there, they too would likely have been isolated for over a billion years.

The possibility that life could have survived for so long locked away from the Earth’s surface turns the researchers’ thoughts elsewhere in the solar system—like Mars. “If such ancient fluids… are preserved deep in the terrestrial crust on billion-year timescales,” they wrote, “perhaps similar potential buried biomes may be preserved at depth in the subsurface of Mars on planetary timescales.” If life did exist on Mars at some point in the past (which is a question we have yet to answer), it might still be there in places where geology and chemistry conspired to create deep, dark refuges.

Of course, that’s a lot of speculation. The researchers are currently looking for organisms in their samples to see if there are a few more secrets they can pry loose.

Nature, 2013. DOI: 10. 1038/nature12127 (About DOIs).