It was discovered in the 1960s that a northern region of Canada has a slightly lower level of gravity than the rest of the world. Roughly, it is 1/10 of an ounce per 150 pounds difference, so it's not huge, but enough for it to be noticed.
Scientists have been arguing since the discovery as to why it has happened. Some said that the underlying mantle rocks are slowly flowing downward. A new study, however, says that during an Ice Age, glaciers pressed down on the area’s crust and the weight of the ice slowly caused the mantle rocks to ooze sideways. Then the ice melted but the crust hasn't bounced back.