Possible Clues:

• Root — one going underground?
• Underground root primarily -- e.g. a potato
• Potato, eg
• Potato maybe in container here when peeled
• Murphy maybe, who rides the underground?
• Potato stem found by Underground user?
• A stalk or a stem
• Rutabaga, for one
• (Dahlia) root
• Fleshy underground stem (eg of the potato)