Under Ground Zone

The Under Ground Zone (not to be mislabeled "Underground Zone") is the first zone of the Game Gear/Master System Sonic 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 like extra lives.

Boss
During Act 3, and in a rather unlikely turn of events, Sonic is saved from a fiery death by Dr. Robotnik, who decides he'd rather watch his spiked adversary die at the mandibles of his latest creation, known as the Arijigoku Mecha in Japanese sources (arijigoku apparently means some sort of giant ant).

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.

Sonic Underground
Under Ground Zone appears in the episode "To Catch a Queen". Dingo (disguised as a man) told Sonic that the Queen was there, however it was a trap.