The Collaborative International Dictionary of English v.0.48:

earthball \earth"ball`\ n.
   any of various fungi of the genus Scleroderma having
   hard-skinned subterranean trufflelike fruiting bodies.

   Syn: earth-ball, false truffle.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

earth-ball
    n 1: any of various highly prized edible subterranean fungi of
         the genus Tuber; grow naturally in southwestern Europe
         [syn: truffle, earthnut, earth-ball]