Volume of ice
2 different figures are floating around:
1) Twice the volume of Lake Michigan
- This is the amount of proven ice in the top metre of regolith around the polecaps
2) Enough for a 500m deep planetwide ocean
- This assumes that the ice we see at the surface is effectively the tip of an "iceberg" that continues to a depth of 5-10 km
I agree that the latter is probable as an interpretation, but it isn't yet proven, which is a pity because once it is, and once people start doing the sums on D/H ratio, the warm and wet early Mars fable is going to unravel.
(Actually, I can think of a way out, but it isn't a real good one)