The resolution of the radar instrument is of the order of hundreds of metres vertically and kilometres horizontally.

It is unlikely that voids of this dimension exist in the shallow subsurface of Mars, but who knows?

The one thing I do anticipate is that this instrument will return some data that seem inexplicable and make us think anew about Mars. Every time we fly a new type of instrument that measures something previously unmeasured, it proves our model of Mars wrong and we have to adjust it a bit, or a lot!

Science is like this, and Mars is a great example. In hindsight, everything is obvious. First time you meet something strange in the dark, it can be pretty scary!