From Exodus3000 Wiki

Isidis Planitia is a flat plain inside a very old crater that extends for about 930 miles (1500 Km) across. It is just north of the equator, on the eastern side of the red planet. The crater was probably created about three or four billion years ago when a comet or a big asteroid collided with Mars. The northern part of the planet is very low and flat. The southern part is much higher and has lots of hills. Isidis Planitia is right along the edge of the southern hills. It is connected to the flat plains in the north. Some scientists think that Mars had oceans a long time ago. If it did, those oceans probably covered the northern plains. Isidis Planitia might have been a bay along the edge of the northern seas.

Isidis Planitia viewed on Google Mars

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