Underneath the rainforest flows a river of magma leading towards a volcanic valley. Watch your step! You don't want to get scorched. And watch your head for falling stalactites! And whatever you do, don't be too reckless! You wouldn't want to cause an eruption, would you?

Up in the mountains lies a temple of worship, long forgotten and flooded from rundown pipes. Traps littered throughout, with puzzles and waterways leading to in-door pools and lost labyrinths.

Dr. Eggman's oil reserve. Use oil geysers to launch yourself higher and higher to free yourself from the bowels of this twisted factory. Make your way outside and witness the pollutants cloud the sky from the sun. Face off against a familiar metallic doppelganger.

A large mineral mine located between the desert and the heavily industrialized areas of the island. Despite the state it's in, its still housing some of the most breathtaking views of the underground world. Shatter crystals and find secret rivers, flowing with iridescent flakes of gemstone. 

Its just New York city mixed with Tokyo and Hollywood. Thats the best way I can describe it.

A fridgid land intertwinned with Robotnik's mechanical prowess. Make your way towards the endgame, you won't get much time to relax.