This article is part of Yahoo's 'On This Day' series.
When astronomer Percival Lowell spotted what he thought were canals on Mars in the 19th century, he sparked an obsession with whether there is (or was) life on the Red Planet.
While Lowell’s discovery made him a laughing stock, today, space probes trundle across the surface of Mars in search of signs of life.
But one of the most dramatic moments was on this day in 2006, when NASA scientists announced the discovery of what appeared to be recently flowing water on Mars, captured by the Mars Orbiter Camera on NASA's Mars Global Surveyor orbiter.
Many scientists believe Mars was once ‘warm and wet’, and that microbial life might have flourished there long ago.
But what NASA announced in December 2006 was much more dramatic: evidence suggesting that water had flowed on Mars within that decade.
The evidence came in the form of bright streaks down the side of two craters, Terra Sirenum and Centauri Montes, which may have been left by several swimming pool’s-worth of water flowing down the gentle slopes.
Due to the low atmospheric pressure on Mars, the water would have boiled away from the surface.
"These observations give the strongest evidence to date that water still flows occasionally on the surface of Mars," said Michael Meyer, lead scientist for NASA's Mars Exploration Programme.
John Murray of the Open University, one of the lead scientists on the European Mars Express spacecraft said, ‘ You've heard of a smoking gun. This is a squirting gun. It is a really interesting and tantalising find. There is so much evidence of past water flow, but if this is right then the same is happening at the present time.
"This is one more place in which we might possibly find life. If you have micro-organisms frozen in water deposits just below the surface of Mars, then yes, these could be revived.
"It's a small possibility but it is a possibility: on Earth, microbescan exist for tens of thousands of years like that and still berevived."
In the decade-and-a-half after the finding, NASA rovers have flown to Mars, and other tantalising signs of flowing water have been uncovered.
In 2015, images from NASA's Mars Reconnaissance Orbiter (MRO) found more evidence that liquid water flows intermittently on present-day Mars.
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Using an imaging spectrometer on MRO, researchers detected signatures of hydrated minerals on slopes where mysterious streaks have been seen on the Red Planet.
Scientists believe that briny water is flowing in a shallow subsurface flow, with enough water wicking to the surface to explain the darkening.
John Grunsfeld, astronaut and associate administrator of NASA’s Science Mission Directorate in Washington said, “Our quest on Mars has been to ‘follow the water,’ in our search for life in the universe, and now we have convincing science that validates what we’ve long suspected.
“This is a significant development, as it appears to confirm that water - albeit briny - is flowing today on the surface of Mars.”
Today, NASA’s new Perseverance probe (armed with its own helicopter) is still searching for signs of life.
Perseverance will explore the Jezero Crater region of Mars.
It will also collect and cache samples of martian rocks and soil for subsequent missions to collect and return to Earth as part of the joint ESA-NASA Mars Sample Return campaign.
Perseverance is armed with X-Rays and ultraviolet light and will zoom in to rock surfaces to look for signs of past microbial life.
Watch: Mars Rover captures stunning images of Martian sand dunes