Giving oxygen to playable character while underwater.
Absorbing the playable character inside them.
Bubbles of the air that rise up in small clusters.
Air Bubbles (気泡,Kihō?) are large bubbles filled with oxygen that appear in the Sonic the Hedgehog series. Air bubbles usually appear in underwater-themedlevels where they are seen periodically rising in small clusters from cracks in the floor. These bubbles provided extra air for the playable character to breathe, and are needed by the player to survive being underwater for longer periods of time. Without using air bubbles underwater, the playable character will eventually drown, thus causing the player to lose a life.
The air bubbles first appeared in Sonic the Hedgehog, and have since then appeared in almost every water-themed level in the Sonic the Hedgehog series. While the air bubbles' purpose and functions have remained the same in all games, there has also been cases in several games where they have aided the player in other ways.
Air bubbles are always found in sections of the Zones that are submerged in water; though, they do not appear in all underwater sections. In these sections, air bubbles can usually be found emerging periodically from the ground in small clusters. In cases such as Carnival Night Zone Act 2 though, air bubbles can also found by popping balloons whilst underwater. Once an air bubble has emerged, it starts rising up to the surface until it reaches it or simply disperses.
When a playable character has to stay underwater for longer periods of time without drowning, the player has to make the playable character make physical contact with an air bubble, before said playable character drowns. This will make the playable character automatically inhale the air bubble and cause it to vanish, thus restoring the playable character's oxygen supply, and extend the time the player can remain underwater. The extended time underwater that air bubbles grant the player is limited though, and the playable character will eventually start to run out of air once more.
While numerous air bubbles of all sizes are always released from their point of origin, it is only the biggest of them that the player can use to restore the playable character's oxygen supply, while the smaller ones goes unnoticed by the playable character. As such, the player often has to wait until a large enough air bubble pops out of the ground to be inhaled.
Air bubbles are not necessarily needed by the player when passing through underwater sections, and a player can make it through a Zone without using them at all. However, because some underwater pathways in the Zones can be so long that the playable character cannot make it through them without getting at least one fresh supply of oxygen, air bubbles are usually necessary for the player's survival underwater.
In numerous games in the series, including earlier main games for the Sega Genesis, if the player is in an underwater section and does not get an air bubble within eighteen seconds, the countdown sequence will start. A small two-note jingles can be heard between five, ten, and fifteen seconds before the countdown sequence. When the countdown sequence begins, the game will display a countdown from five to zero on the screen. If the player does not get an air bubble or get back to the surface before the alarm music ends and counter reaches zero, then a second later, the playable character will drown and the player loses a life.
The alarm music that signifies that the player is about to drown was originally composed by sound programmer Yukifumi Makino, and has since then been used in numerous installments through the series. The following audio track presents how the alarm music sound like in both the 16-bit version of the original Sonic the Hedgehog and Sonic the Hedgehog 2:
Though the air bubbles functions and purpose have remained constant throughout the majority of the Sonic the Hedgehog series, there has been some cases in the games where they have had other functions. In games such as the 8-bit version of Sonic the Hedgehog 2, Sonic Chaos and Sonic the Hedgehog Triple Trouble, there are larger air bubbles that can surround the player. Whilst inside this type of air bubble, the player cannot drown and will float slowly to the surface, while the player can move left and right as well as changing the speed of ascent by pressing up or down. This bubble can be popped by jumping out of it, hitting spikes/hazards and, in Sonic Chaos only, colliding with a wall. Also, in Water Palace in Sonic Generations for the Nintendo 3DS, there is a gimmick where player can rotate the wheel to create large air bubbles for the playable character to bounce on to get to the other side of stage.
Sonic Chronicles: The Dark Brotherhood
Air Bubbles appeared in the last chapter of Sonic Chronicles: The Dark Brotherhood when Team Sonic and Tails enters Perfect Charyb's lair of Nocturne. Perfect Charyb appears and floods the lair with water. While moving around the submerged lair, the player's party will gradually lose POW Points (PP). The player can refill their PP using the air bubbles. Once a character's PP reaches zero, that character's HP will then deplete until it reaches one.
Some stories depict Air Bubbles in flooded zones and locations. In Sonic the Comic Issue 29, Sonic encounters a Sentinel in the story "The Sentinel". Unable to reach nearby air bubbles due to being trapped by debris, the giant stone sentinel rescues Sonic from drowning. Sonic the Comic Issue 83 main story "Bubble Trouble" gives air bubbles a more promident role as Grimer invents a trap that catches Sonic located in the waters of Labyrinth Zone. Sonic was ambushed by new spider badniks that shot fire and fell into the water. After fighting a pair of Spiker badniks, he found a vent of air bubbles where the trap was located and was engulfed by a impenetrable bubble of ultra-tough plastic. This trap similar to the larger air bubbles in the 8-bit Sonic games, albeit with limited air and without buoyancy. He escapes the trap using the flames of the new spider badniks.