The Hudson Bay, and the smaller James Bay (that protrudes south between the Ontario and Quebec provinces) are shown in this image, which was captured by the MODIS on the Aqua satellite on July 21, 2008. Both bodies of water have a great deal of ice in the winter - they are actually a vast inland sea connected to both the Atlantic and Arctic oceans.
The Hudson Bay occupies the southernmost portion of a depression in the land that was created by the weight of a continental ice sheet in the Pleistocene epoch (between 1.8 million and 11,000 years ago). As the ice sheet retreated, the depression was filled with sea water and sediments. With the weight of the ice sheet now gone, the floor of the compressed Bay is slowly rising, and both the Hudson Bay and James Bay are gradually becoming shallower.