Rio Tinto, Spain
This unearthly, reddish river in southwestern Spain is unlike any you've ever seen before. The water gets its coloration from iron dissolved in the water, and is notable for being extremely acidic. This may not sound like a suitable place to find life, but living in the bizarre waters of Rio Tinto are extremophile aerobic microorganisms that feed on the iron and sulphide minerals plentiful in the river.
Scientists now believe that these conditions could be the perfect analogue to what happens when liquid water flows on other planets or moons, such as on Mars or perhaps on Jupiter's moon, Europa. If life can survive under these conditions here on Earth, then it's possible that it also has survived elsewhere, making Rio Tinto an important case study into astrobiology.
In fact, data retrieved from NASA's Opportunity rover has shown that similar conditions to those at Rio Tinto have existed on Mars in the past.