|Aquatic Ruin Zone did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):|
Aquatic Ruin Zone makes an appearance in Sonic Heroes 2. In this game, it is, in part, the submerged remains of Chemical Plant Zone where the plant was left to rack and ruin, in part it is an underwater habitat for tons of natural and artificial wildlife.
In the same vein that Oil Ocean Zone traverses the underneath of Metropolis under the name Aquatic Mine, Aquatic Ruin traverses the bottom of Oil Ocean. However, very much dissimilar to Aquatic Mine, Aquatic Ruin's trespassing is due, in no part big or small, to natural separation of oil and water (oil has a higher density than water). Rogue H2O coaxed the sewage from Oil Ocean upwards and therefore nooks and crannies of Oil Ocean are, like Metropolis', entirely inaccessible via Oil Ocean itself, only via Aquatic Ruin. The same is true for Chemical Plant Zone's Mega Mack; as its density exceeds that of water, the Mack floats, causing areas of Chemical Plant to be accessible via Aquatic Ruin only.