DuckCorp Dico

Found one definition

1.                 From WordNet (r) 3.0 (2006):
                   
   
   
                   
                     
   swallow hole
       n 1: a depression in the ground communicating with a
            subterranean passage (especially in limestone) and formed
            by solution or by collapse of a cavern roof [syn:
            sinkhole, sink, swallow hole]