The temple of Chaos, behind Sonic, Tails and the speech bubbles.
The Temple of Chaos is an ancient Echidna structure on the Floating Island. It was the original home of Knuckles the Echidna's six Chaos Emeralds until he and Dr Robotnik merged them with the six Emeralds taken from the Freedom Fighters in exhange for Porker Lewis' safety. After Robotnik tried absorbing the Emeralds' powers to turn into a God (and was returned to Mobius after having his power drained), the Emeralds were moved into the Emerald Chamber, and the Temple presumably left abandoned.