cracked shower tile/underlayment
If the liner is good you don't need to go down more, though if you replace with the same size tile it may crack in the same place if there's a (evidently) structural issue there. 

I'd be inclined to fill the hole with an "accent stripe" of 1" tile so the crack (when it re-cracks, as seems likely) can follow a grout line rather than crack the face of a tile.