From Sonic Retro
Coco Island (or Kukku Island) is the tenth level in Tails Adventures for the Sega Game Gear, reached via Lake Rocky. While many levels in Tails Adventures require the eponymous fox to return later and acquire new items, Coco Island is once-through only, as the level vanishes after completion.
Apparently the main base of the Battle Kukkus during their invasion of Cocoa Island, this entirely mechanised fortress-isle can only be reached through a long, torturous trial of underwater warfare against the submarine defenses in Lake Rocky. They are, in fact, totally impassable without the starry protection of the Extra Armour upgrade taken from the Mecha Golem in Volcanic Tunnel. But after blasting the Sea Fox through all the guns in the world, Tails hops out onto the steely plating of Coco Island.
A techno-stronghold worthy of Doctor Robotnik himself, Coco Island is filled with conveyor belts, spikes, automatic bomb chutes, laser cannons, activation buttons, electric barriers... the works. Of course, it also includes one feature Ivo wouldn't employ: "Shoukyokuteki Sentouin", the green-feathered Kukku special forces. These sniper birds are impervious to bomb attacks except for the half-second they stand up to snipe at Tails, so you have to time your throws just right. There are many winding mini-passages in this stage which require the Remote Robot to clear.
The boss is the Doctor Fukurokov, a geriatric owl with an immense beard.
Items to pick up in Coco Island include: