The Great Lakes of North America have long been informally called "inland seas," and looking at this animated map of lake currents from the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory, it's easy to see why.
Clicking on the "Latest Surface" observations layer reveals the lakes to be churning pools of motion, at the mercy of prevailing winds and weather systems. But a deeper look shows that, like seas, the lakes have a lot going on below the surface. While the surface currents tend to follow the wind direction, the depth-averaged currents represent the average water motion from surface to bottom and tend to follow shoreline and bottom contours.
Where the Great Lakes differ from true seas is on the matter of tides. While the lakes do experience slight changes in water level caused by gravitational forces, these are overshadowed by the changes caused by wind and barometric pressure.
The map uses technology developed by Fernanda Viegas and Martin Wattenberg at hint.fm for their gorgeous wind map, and is updated four times per day at 3am/3pm and 9am/9pm Eastern Time. Click to zoom in on an area and lose yourself in its mesmerizing whorls and eddies — just try not to fall asleep!