I've found that a decent sized sledge hammer works well (wear your
safety glasses!)

If you're not looking to suplement your weekly workout in this manner,
you could rent/borrow a small hammer drill with a spade bit and chop it
up that way.

If you live in a cold climate, you could do some science experiments
with water poured into the hole and allowed to freeze, and maybe
alternating hot/cold to thermally stress it. These methods will take
some time, but could be interesting to try out if you're into that sort
of thing.