A whirlpool forms when rotational flow builds up in water — either because draining water is forced to converge toward a narrow outlet, or because opposing currents or tides collide and shear against each other, seeding a spin. As that rotating body of water is squeezed inward, conservation of angular momentum forces its tangential velocity to increase toward the center, exactly like a figure skater spinning faster by pulling their arms in. In the open ocean, large slow-turning whirlpools can also be shaped by the Coriolis effect over hours or days, though at everyday scales (like a bathtub drain) Coriolis is far too weak to matter. Famous real-world examples include the Moskstraumen (the "Maelstrom") off Norway's Lofoten Islands, Old Sow in the Bay of Fundy near New Brunswick, and the Naruto whirlpools in Japan's Naruto Strait, all driven by strong tidal currents funneled through narrow channels.

• Naruto's tidal whirlpools can reach current speeds of roughly 13 km/h (8 mph) and whirlpool diameters up to about 20 meters.
• A draining bathtub demonstrates the same core physics as a giant ocean whirlpool, just at a tiny scale.
• Myth: bathtub drain direction is NOT reliably set by hemisphere/Coriolis — that effect only becomes significant at large scale and over long timescales, not in a small basin.
• Angular momentum conservation (v × r ≈ constant) governs the speed profile: velocity rises sharply as radius shrinks toward the core.
• Strong whirlpools can snag and pull in floating debris, and historically posed real hazards to small boats.
• The central low-pressure core and downward funnel shape are why light foam and debris tend to collect and swirl near the rim before being drawn down.