The bay extends from Cape Ann on the north to Plymouth Harbor on the south, a distance of about 42 miles (68 km).

Its northern and southern shores incline toward each other through the entrance to Boston Harbor, where they are about five miles apart.

The depth from the base of the triangle to Boston Harbor is about 21 miles (34 km).

The Massachusetts Bay Disposal Site in deep water off the coast has been used for ocean dumping, to dispose of munitions, dredged material, rock and construction debris and sunken vessels.

Fishermen have brought a torpedo into Provincetown, a depth charge into Gloucester, and mustard munitions into New Bedford.