This is an image showing a Martian outflow channel.
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Image from: NASA

Martian Outflow Channels

The Martian surface shows evidence of the remains of a large number of tear-drop shaped lakes such as the one shown in this image. Most of these are found on the slopes of the Martian volcanoes. They are really channels where water once gushed forth. They seem to start from a hole in the ground, as if whatever came out of the ground came out full steam. The channels are typically 10 m to 100 m deep (about the depth of a football field), 40 km across, and filled with boulders. Mars Pathfinder landed on a channel such as this called Ares Vallis.

The most likely explanation for these is that they were formed when water near the surface erupted out of the ground. There is no water left in these lakes today.

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