Hydrocity Zone/Pre-SGW

Hydrocity Zone, more commonly known as Hydro City, is an ancient and abandoned underground city filled with water located underground on Angel Island. Constructed hundreds of years ago, it was supposedly constructed even before Echidnaopolis had been.

Appearances and Mentions
In 3236, Knuckles the Echidna lead Monk to Hydro City to evade being captured by Hunter. Eventually, Knuckles managed to defeat the Overlander there, but not before Monk was killed. (KtE: #32)

When a trio of Scout-Bots arrived in the Chaos Chamber on the Floating Island in 3228 seeking out a power source of their ship, Locke told them they could use solar cells from Hydro City, not the Chaos Emerald. (StH: #116)

Background Information

 * Assuming Hydro City was built before Echidnaopolis, it would had to have been built under the ground and before the Floating Island was created.
 * Hydro City used solar cells as a form of power.
 * This area seems to be named after the Hydrocity Zone from Sonic the Hedgehog 3. However, here it looks more like a sewer than the ancient city design the games give it.