A great ocean current transporting about 7 × 10⁷ tons (6.3 × 10⁷ metric tons) of water per second (1000 times the discharge of the Mississippi River) northward from the latitude of Florida to the Grand Banks off Newfoundland. Before the days of scientific oceanography, it was supposed that the origin of the water in the Gulf Stream was the Gulf of Mexico. The origin has since been traced farther upstream, through the Gulf of Mexico and Caribbean, to the Great North and South Equatorial currents of the Atlantic. The Gulf Stream is thought of now as a portion of a great horizontal circulation in the ocean, where particles of water execute closed circuits, sometimes moving slowly in mid-ocean regions and other times rapidly in strong currents like the Gulf Stream. Thus the beginning and end of the Stream have arbitrary geographical limits.