Working about a mile and a half under the surface in a deep Ontario mine, a team of scientists have discovered a pocket of water which they estimate to have been trapped underground for more than a billion years! It is unknown if any microbial life has been residing in the water all this time, but the right gasses to support life – methane and hydrogen – have been found. Through analysis, the scientists determined the fluids have not made contact with the Earth’s atmosphere for at least a billion years.
An important implication of this finding is on Mars. The surface of Mars is known to have contained water at one point in its past, and the rocks are chemical similar to Earth rocks. While the idea is entirely speculative at this point, microbial life may exist on Mars in these type of underground caves or reservoirs.
Source: http://www.nature.com/news/reservoir-deep-under-ontario-holds-billion-year-old-water-1.12995