Lava Reef Zone

From the Sonic News Network, the Sonic Wiki

Lava Reef Zone is the fourth level of the game Sonic & Knuckles. It follows the Sandopolis Zone and precedes the Hidden Palace Zone. It is based in a boiling hot network of mines, and then the magma chamber of the Angel Island volcano in which the Death Egg crashed at the beginning of the game.

There are lots of tear away platforms, and most of them break away into bubbling lava. As in the Volcano Valley Zone in Sonic 3D, the most valuable commodity here is the Fire Shield. The mini-boss is a pair of bullet-shooting tentacles and a giant metal hand. To make the fight go faster, destroy the tentacles first.

In the second act, a majority of the boiling lava has hardened. If playing as Sonic, Knuckles will drop you into a cavern with the Death Egg for the boss fight. The Death Egg will turn on and cause lots of lava to appear. The first part is just a survive for a set amount of time part. After that, you will be going down a lava fall. Find the Flame Shield and jump down to fight the boss. You can't hurt it, but its own mines will. Defeat him to move on to Hidden Palace Zone.