Hydro City

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. In 3236, Knuckles the Echidna lead Monk there to evade being captured by Hunter, and eventually Knuckles managed to defeat the Overlander there, but not before Monk was killed. (KtE: #32)
 * Assuming Hydro City was built before Echidnaopolis, it would had to have been built under the ground and before the Floating Island was created.