International law uses a semicircle to decide what counts as a bay
Draw a line across the mouth of a coastal inlet, then imagine a half-circle on that line. Under the UN Convention on the Law of the Sea, the inlet is a legal bay only if its water covers at least that much area. Anything shallower is just a bend in the coast, called a bight.
A bay is a recessed stretch of coastal water joined directly to a larger body, whether ocean, lake or another bay. Size and shape bring different names. Big ones may be called gulfs, seas, sounds or bights; a cove is a small round bay with a narrow entrance; a fjord is a long bay carved by a glacier. Bays can nest inside each other, as James Bay does within Hudson Bay in northeastern Canada, and a bay can be a river's estuary, as Chesapeake Bay is for the Susquehanna. Gulfs such as the Gulf of Mexico and the Persian Gulf are typically larger, often with narrower mouths.
The legal definition carries real stakes. A bay that meets the criteria counts as a state's inland waters rather than international waters, which affects rights to the seabed and its minerals, control of fishing, whether ships may pass freely, and even whether a coast forms an international border. Geographers also classify bays by shape: open bays widen toward headlands at the mouth, enclosed bays have narrower mouths behind a peninsula, and back-barrier bays sit behind barrier islands or spits.
The largest bays are products of plate tectonics. When the supercontinent Pangaea broke apart along curved fault lines, drifting continents left behind huge indentations such as the Bay of Bengal, the Gulf of Mexico and West Africa's Gulf of Guinea. The Bay of Bengal, which at about 2.6 million square kilometres is the world's largest bay. Smaller ones come from erosion: glaciers gouge fjords, while rivers produce rias with gentler slopes. Where soft rock meets hard, the soft erodes into bays and the hard survives as headlands.
Surrounding land blunts the wind and blocks waves, which made bays attractive to early settlers seeking fish and, later, to traders needing safe anchorage. Many of the world's ports grew up in them for exactly that reason.
Source: Bay
