Repairing the Death Egg
After his defeat at the hands of Sonic and Tails, Dr. Eggman's Death Egg crash landed on the floating island, resulting in the island sinking down to the water's surface. Eggman used the Launch Base Zone as his base for repairs and apparently messed with the systems. Hydrocity Zone was the area that Sonic and Tails passed through after Angel Island's beach and forest area, which eventually lead to Marble Garden Zone. (VG: SH3aK)
Knuckles later passed through Hydrocity Zone to reach Marble Garden Zone in pursuit of a fleeing EggRobo controlled by Dr. Julian Snively, Dr. Eggman's assistant, after multiple encounters on the island. (VG: S3aK, SU: #70)
- This area is based on the Hydrocity Zone from Sonic the Hedgehog 3.