Now, NASA scientists believe that a large lake in the Gale Crater — or even a series of lakes that evaporated and then reformed — caused the mountain's unusual formation.

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To have liquid water on the surface, Mars would have had a much warmer, heavier atmosphere than it does now. NASA scientists still aren't sure how that atmosphere formed, or why it changed. But based on Curiosity's readings around the Murray formation — a section of rock 500 feet high — it seems that the crater lake filled with sediment (carried in by rivers) over and over. Once this sediment reached a certain height, the hardened sediment was eroded by wind, eventually forming the mountainous shape we now see.

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"The great thing about a lake that occurs repeatedly, over and over, is that each time it comes back it is another experiment to tell you how the environment works," Curiosity Project Scientist John Grotzinger said in a statement. "As Curiosity climbs higher on Mount Sharp, we will have a series of experiments to show patterns in how the atmosphere and the water and the sediments interact. We may see how the chemistry changed in the lakes over time. This is a hypothesis supported by what we have observed so far, providing a framework for testing in the coming year."