Why don't soda bubbles go down?

Soda bubbles are full of gas. (In fact, that's all they are.) The gas in the bubbles is less dense than the soda surrounding them, so they float up.

In layman's terms, the bubbles are full of gas that is lighter than the liquid and that makes them float. Sometimes you see people sticking their fingers into the foam to keep it from overflowing. This makes the bubbles disappear suddenly, because when the natural oil on our skin touches the bubbles and spreads across them, it breaks them down.