In the 1960s, scientists began to chart Earth’s gravitational fields. You would expect that gravity be the same across the world. It’s not like you feel incredibly heavy in the United States while you practically float around in South America. You always feel the same gravitational pull.
However, they found that gravity actually did vary. In Canada, there
are certain areas with less gravity than normal. The Hudson Bay, for
example, is “missing” gravity.
There are two theories as to why this may be true. One theory cites
convection as the cause. Convection pulls the Earth’s plates downward,
which decreases the mass in that area and decreases the gravity. The
other theory cites the Laurentide Ice Sheet as the cause. The Ice Sheet
melted 10,000 years ago and left a huge indent in the Earth, which could
mess with the gravity.