After decades of looking, a NASA spacecraft has found evidence of liquid water on Mars. Billions of years ago, Mars was flowing with water, but for the past few decades that we've been researching the red planet, the only H2O to be found was in the air, soil, and the ice in the planet's poles. Five years ago, however, researchers noticed "mysterious dark streaks running down the warm slopes of Martian craters and mountains." They entertained the possibility that this could be water. To test this hypothesis, they sent a spacecraft, which noticed "deposits of perchlorate" that were being moistened by a compound none other than water.

Sources: USA Today | NYTimes | NASA