Sonic the Hedgehog 2 (8-bit)
Hop on a runaway railcart for the ride of your life through showers of spikes and over daredevil jumps. Buzz-saw your way through to secret tunnels and hidden Monitors. Sometimes it's hard to get enough momentum for a good ground attack, but get a running start and keep trying!
— Description from the Instruction manual.
Under Ground Zone (not to be mislabeled "Underground Zone") is the first Zone in the Master System and Game Gear game Sonic the Hedgehog 2. As with all other stages in this game, Under Ground Zone consists of two standard Acts followed by a shorter, third Act which contains the Zone's boss.
It is a hot, rocky Zone filled with rail carts which Sonic can ride, and magma pits which the rail carts invariably dump you in. Craggy mountains rise in the background and there are a number of spin-dash-able walls leading to hidden goodies such as extra lives.
During Act 3, and in a rather unlikely turn of events, Sonic is saved from a fiery death by Dr. Robotnik in his Egg Mobile, who decides he would rather watch his spiked adversary die at the mandibles of his latest creation, known as the Antlion Mecha.
Sonic has to dodge a variety of fatal bouncing rubber balls, while simultaneously avoiding the robotic claws that lie in wait at the bottom of the pit.
This boss is generally considered to be a lot more difficult in the Game Gear version of the game, as the smaller screen size for that system drastically reduces the time a player has to get out of the way of the descending kill-balls.
In other media
In the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics, the Under Ground Zone (here spelled Underground Zone) is a region on. It was here where Sonic the Hedgehog first met Breezie the Hedgehog in his mission to save Tails from Dr. Eggman.