NASA's curiosity rover has sent back images of what is believed to be an ancient stream bed.
Below is a picture comparing Curiosity's photo (on Left) to a picture of an ancient stream bed from Earth (on right).
Scientists believe that the similarities between the two landscapes are enough to conclude that at one point there was a stream on Mars. Scientists also believe that this was a fast moving stream that was also probably deep.
NASA's Jet Propulsion Laboratory blog reported the significance of of this find:
"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," said Curiosity science co-investigator William Dietrich of the University of California, Berkeley. "Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it."