Wet enough to create layers of sedimentary rock. Gale Crater, for example, appears to have contained a lake that, over time, varied in size.
And apparently for it to exist, there had to have been seas. It's now looking as if Mars was once quite habitable by our kind of life.
We can't work out how. None of the models we have allow Mars, with its small size, to have a thick enough atmosphere to be warm enough for what the rocks are telling us happened.
Unless, of course, the NASA researchers are wrong. We'll see what ends up happening, but there's still a lot to learn.
And a lot, apparently, that we don't quite understand about the Red Planet.