It uses an acrylic container thereby making an impression that is a solid volum of water. As to how the circular movement comes about around, it’s all magic, er, physics.
The sirens Charybdis and Scylla resided in the Sicilian Sea. Homer tells us that because Charybdis had stolen the oxen of Hercules, Zeus struck her with a thunderbolt and changed her into a whirlpool whose vortex swallowed up ships. In Charybdis the circular movement of water inside a transparent acrylic cylinder forms an air-core vortex in the centre. Steps wrap around the cylinder and allow spectators to view the vortex from above. The cylinder was manufactured in Grand Junction, Colorado.