EAST LANSING (WKZO) -- The 1975 disappearance of Jimmy Hoffa remains one of Michigan's most baffling mysteries, and it will continue that way.  Experts from Michigan State University said yesterday they found no traces of human remains in soil collected after Roseville police dug up part of a yard under concrete on Friday.  Officers were acting on a tip that a body may have been buried there around the time the Teamsters boss vanished.