Saltier than sea water and the consistency of "very light maple syrup." Yuck.
Last month, a paper published in Nature reported on some water that had been trapped 1.5 miles below the Earth's surface in Canada for a long while. How long? Based on an analysis of the isotopes of natural gases in the water, scientists believe it to be the oldest isolated water ever studied, at least 1 billion years old and maybe as old as 2.64 billion, slightly younger than the rocks that encased it.* For maybe half as long as the Earth's entire existence, this water has been sealed away, unexposed to the atmosphere.
The next question: Is it drinkable? The answer: Not really, but a sip won't kill you. According to an interview in the Los Angeles Times, one of the paper's authors, Barbara Sherwood Lollar, has tasted it, and it was "terrible," she reports. "It is much saltier than seawater."
Her description of its appearance doesn't make it sound very appetizing either:
For the rest of the story: http://www.theatlantic.com/technology/archive/2013/06/confirmed-1-billion-year-old-water-tastes-terrible/276946/