Lava Reef Zone is the fourth Zone of Sonic & Knuckles (and the tenth of Sonic 3 & Knuckles). It is the longest Zone in Sonic & Knuckles, and is also the longest zone in any other Genesis Sonic game. 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 Floating Island's 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 Blast, the most valuable commodity here is the Fire Shield. The mini-boss is a pair of bullet-shooting tentacles and a giant metal hand. Destroying the hand actually ends the boss fight, even if the tentacles are not yet destroyed.
Act 2 of this level is the player's last chance to obtain Chaos Emeralds/Super Emeralds, and there are only a few warp rings in the Zone. In this second act, a majority of the boiling lava has cooled and hardened. As the player makes his/her way through the massive cavern, the Death Egg becomes visible in the height of the mountain, still inactive. If playing as Sonic, Knuckles will drop the player into a cavern with the Death Egg for the boss fight. The Death Egg's laser cannons in its eyes suddenly activate, causing the lava to reach flashpoint and return to its molten state. Some of the ground actually turns fragile at this point and Sonic must immediately move before facing a hot bath in the bubbling lava below. Dr. Robotnik appears in his fortified Egg Mobile, armed with missiles and mines. The player must make his/her way from platform to platform ahead of Robotnik's missile barrage and avoid falling to his/her death as the screen forcibly progresses. After the camera halts, the player must traverse sparse footing down a lava fall. There's a Lightning Shield here which can help collect all the rings, but the Flame Shield is by far the most valuable for the boss fight. Find the Flame Shield and jump down to fight the boss.
In a similar fashion to the Submarine Eggman boss from Hill Top Zone in Sonic the Hedgehog 2, Robotnik will rise up from the lava. However, his machine is fully protected with spikes. On the center platform, Robotnik will appear on the right and left sides, rising from the lava. Creating a southwest/southeast current, he'll attempt to make the platforms Sonic/Tails are standing on run into his protected machine, while constantly shooting spiky mines in means to cause problems while the player jumps from platform to platform. Eventually, after enough mines hit his machine, the scientist will be defeated and the lava will cool down, allowing access to the Hidden Palace Zone.