Sega released Sonic & Knuckles for the Mega Drive and Genesis in 1994, completing the story that began in Sonic the Hedgehog 3. After discovering that Dr. Robotnik is after the Master Emerald, the source of Angel Island’s power, Sonic continues his chase while Knuckles finally becomes playable in his own campaign. Knuckles can glide through the air and climb walls, which opens up alternate routes and areas that Sonic cannot reach. Zones include Mushroom Hill, Flying Battery, Sandopolis, Lava Reef and Sky Sanctuary, ending with a battle aboard the Death Egg. The cartridge itself featured Lock-On Technology, a slot on top that accepted other Sonic games. Connecting Sonic 3 created the full Sonic 3 & Knuckles adventure, while connecting Sonic 2 let Knuckles play through that game. It remains one of the most inventive releases of the 16-bit generation.