A fluid that consists of a thin layer lying on top of a thicker and denser layer of water is a laboratory model of the upper ocean. Water at the surface of the ocean is usually warmer than water in the depth and this variation of temperature results in a density difference. Different forces acting on the surface of the ocean generate vortices. A wind pulse, for example, can induce a vortex dipole or a mushroom-type flow. A vortex dipole is a jet with a system of two vortices of opposite signs at its front. It resembles a sliced mushroom - hence the name. The flow in the photograph shows several dipoles induced by a small object moving in the upper layer. The flow was made visible with a pH indicator in an acid/base water solution rather than a regular dye. The areas of fluid that were initially of different acidity are colored differently and, when deformed by the flow, show the pattern of vortices.