What did Mars look like when it had oceans and an atmosphere?

Mars is known as the Red Planet. Why is it red? In short because the planet hasn’t seen a drop of water in billions of years and it’s a giant and perpetual dust bowl–much like the American Midwest experienced in the 1930’s. 

It is common knowledge that Mars had oceans like Earth in its distant past. So what would this neighboring planet look like if it again had water?

Kevin Gill, a software engineer,  pursued this question through his project, A Living Mars

Here’s his result: