What is the difference in tunneling and undermining in a wound

Subtle difference: Only real difference is appearance. Both involve extension of the wound beneath the skin. Tunneling is generally narrow, and in one direction, while undermining is below any side of the wound.

They are similar: A wound with undermining means that the wound is larger than the skin defect, ie the deep aspect of the wound extends beyond the skin edges. This can sometimes be a rather large area. Whereas tunneling typically refers to an area of undermining under the skin that is long and narrow, creating a "tunnel" in 1-2 directions, rather than all dierections.

Extent: Tunneling means that a small cavity is present, think that a Q tip could be pushed into the body, into a void like a cave in the tissues. Undermining is similar in that there is a void but much bigger. Undermining surrounds an opening at the skin level, there is no tissue present beneath a significant area of skin to support it.