Countdown

The Countdown is played when Sonic (or any other character) is in water for too long. The player has 18 seconds to try and get air or the character drowns.

Trivia

 * Early on, the countdown had a zero at the end. This was scrapped (possibily in Sonic Adventure).
 * Air Bubbles are sometimes found (but not all), in some Sonic games. These are helpful to the player as they can get a quick breath of air so the character can stay underwater longer.