A block of dry ice is placed in the bottom of a clear plastic cylinder sealed on the bottom, trapping carbon dioxide in the bottom of the container as the dry ice sublimates (evaporates). Bubbles produced with a standard soap bubble blowing gizmo will float on top of the more dense carbon dioxide. Some carbon dioxide diffuses into the bubbles, so they get larger and sink! Some of the bubbles freeze when they sink to the bottom of the container near the dry ice. The photograph at the right is a detail of the top of the container.
It may be necessary to remove static electricity from the plastic container. A damp cloth or lit match may assist with this.