In an effort prove the existence of the Coriolis Effect as it presents itself in the toilets of the Northern and the Southern Hemisphere, Veritasium host Derek Muller in Australia and Destin from Smarter Every Day in Alabama, conducted identical experiments from across the world. With deliberate controls in place, they each filled a kiddie pool and then drained it slowly, marking each direction with food coloring to determine which way the water drained. The pool in Alabama drained counter clockwise, while the pool in Australia drained clockwise.
The idea that water going down a drain or flushed down a toilet swirls in opposite directions in the Northern and Southern Hemispheres has a long history. But few have ever done the experiment. Destin from Smarter Every Day and I performed identical experiments in the Northern and Southern hemispheres. What we found is the direction of water swirl in a toilet, sink, or bathtub is determined by other sources of angular momentum. However if the body of water is big enough, e.g. a kiddy pool, and left still for long enough (at least 24 hours), then the Coriolis effect is observable with water swirling counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere.