Maʼadim Vallis is one of the largest outflow channels on Mars, about 700 km long and significantly larger than Earth's Grand Canyon.

It looks as if water may have collected in Gusev crater, forming a giant lake; the Spirit Rover was sent there to investigate that possibility, but found only volcanic rocks on the floor of Gusev.

[3] Maʼadim Vallis is thought to have been carved by flowing water early in Mars' history.

Sapping occurs when groundwater partially dissolves and undermines the rock, which collapses into debris deposits and is carried away by other erosion processes.

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