Source : Webster's Revised Unabridged Dictionary (1913)

Molehill \Mole"hill`\, n.
   A little hillock of earth thrown up by moles working under
   ground; hence, a very small hill, or an insignificant
   obstacle or difficulty.

         Having leapt over such mountains, lie down before a
         molehill.                                --South.

Source : WordNet®

molehill
     n : a mound of earth made by moles while burrowing