The Power Chamber in the Underwater City.
The Power is the prosaic term given to a source of Chaos Energy, similar to the Chaos Emeralds, that fuels the Underwater City. Rather than the giant gemstones of the Emeralds, the Chaos Energy ensorcelled within The Power instead imbues some sort of bubble or smooth rock. This is evidently an inferior source of containment to the Chaos Emeralds - while those can be safely handled unless specifically tapped for energy, The Power continually radiates Chaos Energy into the environment - as a vivid barometer of its polluting effects, merely being in the presence of The Power was sufficient to begin transforming Sonic into Super Sonic, and mechanical technology (presumably the Underwater City's own devices have different characteristics) failed inside of a radius of a few miles.