The Egg Bubble is the third boss encounter in Sonic the Hedgehog CD. As with most of Dr. Eggman's boss machines on the Little Planet, Tidal Tempest's boss encounter is heavily tied to the level theme - namely, the Underwater theme.
Sonic the Hedgehog, fresh from getting through the first two water-filled Zones, starts off by chasing the doctor as he rides about in his trusty Egg Mobile, echoing the Labyrinth Zone boss from the first game in the series. However, instead of trying to climb out of the labyrinth, the crafty scientist instead leads Sonic deeper into the maze-like area. At certain parts Sonic must be curled up in his spin attack as he falls, otherwise he will be open to attack, unable to defend himself from the moving Eggmobile. The chase continues to loop through the level until the player is able to land four hits on the egg-shaped doctor, who begins to panic, opening a doorway towards another area of the level.
Once through, Sonic lands in a water-filled cavern, and although a power-up may be hiding out on the left, the right leads to the true boss encounter of the zone. Still within his Eggmobile, Dr. Eggman pulls numerous air bubbles around his ship, circling him in a similar fashion to the Flying Eggman boss of Sonic the Hedgehog 2. However, instead of having seven twirling items that can cause pain, the numerous bubbles that surround Eggman actually serve a purpose for the player, allowing Sonic the Hedgehog to breathe during the fight that takes place completely underwater. Eggman is not without his defenses, four energy balls shooting from the bottom of the craft as he either slides across the screen or swoops downward in an attempt to crash into Sonic. The only way to defeat the crazed scientist is to land a direct hit against the Eggmobile, made difficult by the number of air bubbles. Though Sonic can continue popping them, each pass across the screen allows Eggman to redistribute the remaining bubbles, filling any holes that may have been made only a second beforehand. However, Eggman's supply is limited, and eventually a gap will open up that even he can not fix, letting Sonic land the final blow. With the machine exploding, Eggman scurries away, the water flowing away to let Sonic breathe easy as he makes his way to Quartz Quadrant.
Sonic preparing to chase Dr. Eggman into the heart of Tidal Tempest.