The man had jumped from a plane but his parachute had failed to open. It is the unopened package.

This is sometimes given with the following rather elegant clue - as he approached the centre of the field he knew he was going to die. This is another of the top classics which is right up there with 'The Man in the Bar'. If the solver is thinking along the wrong lines (i.e. in the two dimensions of the ground) then the lateral jump to the third dimension can be tough to make.