The top image shows the radar imaging of underground structures on Mars, below that is the typography of that same region and the radar swath is represented by the white line that cuts through it.
In the radar image, the point at which the traces or “layers” split into two (on the left side) is where it is thought that the materials change and so the radar is echoing again off that lower subsurface. They also add that “the strength of the lower echo suggests that the intervening material is nearly pure water ice”, which apparently may translate into a depth of 3.5 kilometres of ice. This image is not alone, additional radar scans have suggested ice that runs to a depth of 1.5km in other places with surely many more to come.
It would seem that the intended purpose of finding where, or if, water exists on Mars has more or less been satisfied. I don’t think that anyone expected to find this much evidence of water on Mars between the recently discovered water flows and now a nearly planetwide (theoretical) underground reservior of pure water ice. Future astronauts on Mars will not go thirsty!