We've all heard that water spirals down the drain in a clockwise direction in the Southern hemisphere and counter- (or anti-) clockwise in the Northern hemisphere, due to the Coriolis effect from the rotation of the Earth. I've also heard that this is a myth, and that if you calculate the actual forces involved, those due to Coriolis are negligibly small compared to the random perturbations that typically occur in a basin of water that was filled relatively recently (or otherwise disturbed). The latter sounds likely to me, yet the "myth" persists, and moreover, you can see demonstrations of it on YouTube: . In this video, it appears that the water behaves as advertised, even though the basins are displaced from each other by no more than 10 meters or so. Yet, the one on the North side of the equator drains clockwise, the one on the South the opposite, and most amazingly, the one right on the equator doesn't swirl at all, but just drains straight down the hole! How can this be? I thought the Coriolis force depended on a change in the radial component of position, but in these cases it's hard to see how that's not completely negligible.