Flying Battery Zone

Flying Battery Zone is the second zone of Sonic & Knuckles and the eighth zone of Sonic 3 & Knuckles, taking place after Mushroom Hill Zone and before the Sandopolis Zone.

This zone takes place on Dr. Robotnik's gigantic metal blimp. The blimp is powered by rotating drills and machines that send out worm like bridges. Flame jets can act as springs if the player stands on one for long enough, the pressure sending the character upwards; sometimes they even act as hidden switches. The mini-boss, Gapsule, is a machine that looks like one of the animal capsules. It has two huge ball-and-chain arms. The player needs to stand on the switch on top of its head and wait for it to rear back and swing. Then s/he needs to get off the machine and go out on the side in order for the machine to damage itself.

Despite the zone being one of Robotnik's contraptions, there are some Warp Rings in the level, which are accessed by pushing certain groups of spikes.

The second act basically starts out the same, but with slightly different music. There's an area where the player needs to survive by dodging a beam for a set amount of time until it blows itself up for unknown reasons (a similar machine is used in Sonic 2's Wing Fortress Zone, except that there, you needed to attack the laser, and the walls don't close in). After that, you need to race to the top of the blimp without getting crushed by the moving floor. At the end of the level, Robotnik awaits in a modified Eggmobile. It holds itself onto the floor the player stands on with clamps with spikes around, and occasionally swings himself over and back under the ground multiple times, which is where the player gets a chance to attack it. When standing above Robotnik while he's below the ground, he sometimes shoots fire upwards. Once defeated, the player character escapes into Sandopolis Zone.

Trivia

 * According to Sonic 3's (alone) Sound Test option, this was supposed to be the fifth level in the game. Sonic (or Knuckles) was supposed to see the blimp in the carnival and be shot through the cannon (or in Knuckles's case, be transported) to the blimp. At the end of the zone, Sonic was supposed to break the door of the battery blimp and use it as a snowboard in the Ice Cap (Knuckles wouldn't get this scene because he doesn't normally snowboard). However, at that point of development, a proper transition from Mushroom Hill to Sandopolis probably wasn't thought of, so the developers decided to place Flying Battery between the two. The Sound Test option in both Sonic & Knuckles and Sonic 3 places Flying Battery's music after Sounds #7 and #8 (Carnival Night's BGM). There's even an unused icon of Flying Battery leftover in Sonic 3 that was supposed to appear between Carnival Night and Ice Cap. Another possible explanation for Flying Battery being placed in Sonic & Knuckles was to keep the levels between Sonic 3 and Sonic & Knuckles even.
 * Doctor Robotnik appears in the boss battle for both Sonic and Knuckles, despite Knuckles' boss battles usually being against EggRobo. The sprites for the pilot character were different for this boss battle than for any of the others, facing to the screen instead of side-on. No sprites were provided for EggRobo in this new position (for unknown reasons), so Robotnik's were used instead.
 * According to fans, this flying fortress of metal is used to carry animal capture capsules. The line of capsules in the background of this stage provide evidence to this theory.
 * The Flying Battery attacked Sonic and Tails in Angel Island Zone before the boss.
 * From outside, it resembles the ship shown in the Sonic Generations' remake of Rooftop Run. For a part of rooftop run after opening the outside of the clock in Spagonia and which the Battery Blimp crashes into it, Classic Sonic boards the Flying Battery zone alike ship goes down into a part of it and runs out of it while it starts to burn.