(Photo: Atlas Obscura)
The Saint Croix River forms part of the boundary between the US state of Maine and the Canadian province of New Brunswick. The river empties into Passamaquoddy Bay, where it encounters a strong Atlantic current. Where the two opposing currents meet--slightly over the Canadian side of the sea border--you can find Old Sow. It’s the largest tidal whirlpool in the Western hemisphere.
You can visit, but you should ask local boatmen when it’s active. Old Sow is a tidal whirlpool, so it appears only during the right tidal conditions. If you take the ferry from Deer Island, New Brunswick to Eastport, Maine, you may get a good look at it. You can also take a boat and earn an official certificate from the Old Sow Whirlpool Survivors’ Association.