On the northern hemisphere of Mars, in the region called Hephaestus Fossae can be seen mysterious ancient river which was most probably interrupted by some kind of meteorite. The remains of this can be seen on the image above (credit ESA, click here for full resolution). The image was taken by Mars Express orbiter few years ago. The colours on image are describing the deep areas and the terrain itself.
The deepest place here is 4 km deep. Scattered across the scene are a few dozen impact craters that cover a wide range of sizes, with the largest boasting a diameter of around 20 km.
Based on the lack of structures near the small craters in this image, scientists believe that only the most powerful impacts – those responsible for forging the largest craters – were able to dig deep enough to release part of the frozen reservoir of water lying beneath the surface.