Which is the world's largest bay by shoreline?

Hudson Bay, with a coastline of something over 12,000 kilometres and an area of about 1.2 million square kilometres - large enough that calling it a bay rather than a sea is largely a matter of convention. It is shallow, averaging around 100 metres, and freezes over completely most winters.

The land around it is still rising. The Laurentide ice sheet was thickest here, pressing the crust down by hundreds of metres, and since it melted the ground has been rebounding at up to a centimetre a year - fast enough that the bay is measurably shrinking, shorelines are stranded inland, and the region has the lowest gravity anywhere on Earth, because the mantle has not yet flowed back underneath it. Satellites can see the dent.

It also carried a great deal of history for something so empty. The Hudson's Bay Company was granted a trading monopoly in 1670 over the entire watershed - about 3.9 million square kilometres, roughly a third of modern Canada, handed to a group of London investors by a king who had never seen it. The company effectively governed that territory for two centuries and still exists, which makes it one of the oldest continuously operating commercial enterprises in the world.