When digging in a mine, the bottom of the screen will display vital information:

Pick
The amount of pick left for that mine.
Fighting monsters will deplete pick hit if the shield is completely drained.
The gear equipped will have an impact on this number.
Shield
This represent the amount of hit you can take from monster.
Once this is depleted, fighting monster will remove pick hits instead.
The gear equipped will have an impact on this number.
Key
Each key can open one chest in the mine.
The gear equipped will have an impact on this number.
Depth
This is the numerical value of your current depth in relation to the total depth of the mine.

Pick
The amount of pick left for that mine.
Fighting monsters will deplete pick hit if the shield is completely drained.
The gear equipped will have an impact on this number.
Shield
This represent the amount of hit you can take from monster.
Once this is depleted, fighting monster will remove pick hits instead.
The gear equipped will have an impact on this number.
Key
Each key can open one chest in the mine.
The gear equipped will have an impact on this number.
Depth
This is the numerical value of your current depth in relation to the total depth of the mine.