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.