Vortex rings in water

Vortex rings can be created not only in air (see the "Vortex rings in air" section) but also in water. The best way is to immerse a bottle filled with water and press it strongly. The rings can be visualized by placing some source of bubbles outside the bottle opening, for example the air pump for aquariums, or by simply making many bubbles in the water after adding some detergent. The air bubbles arrange along the vortex line where the pressure is lower because of the fast rotation. The best way to observe such vortex rings is to do so in a sea using a diving mark and a snorkel.