Here is a bit more on two theories about the weird gravity effect in the Hudson Bay area of Canada:

One theory centers on a process known as convection occurring in the Earth’s mantle. The mantle is a layer of molten rock called magma and exists between 60 and 124 miles (100 to 200 km) below the surface of the Earth . Magma is extremely hot and constantly whirling and shifting, rising and falling, to create convection currents. Convection drags the Earth’s continental plates down, which decreases the mass in that area and decreases the gravity.

A new theory to account for the Hudson Bay area’s missing gravity concerns the Laurentide Ice Sheet, which covered much of present-day Canada and the northern United States. This ice sheet was almost 2 miles (3.2 km) thick in most sections, and in two areas of Hudson Bay, it was 2.3 miles (3.7 km) thick. It was also very heavy and weighed down the Earth. Over a period of 10,000 years, the Laurentide Ice Sheet melted, finally disappearing 10,000 years ago. It left a deep indentation in the Earth.

We hope you like the video and feel free to comment on which one is the most surprising.

thanks to howstuffworks.com for the great info