Undermining (also known as Removal of the Guard) is a chess tactic in which a defensive piece is captured, leaving one of the opponent's pieces undefended or underdefended. The opponent has the unpalatable choice between recapturing or saving the underdefended piece. When using this tactic, however, one should keep in mind that the opponent can sometimes sacrifice the piece whose defence is being undermined before capturing the piece which just took the defender.

to tunnel at the corner of a square stone keep to upset the foundations so that the castle wall collapses.

Digging out the substance that lay beneath the seam of coal so that the coal would drop down

separating skin and fat from underlying tissue