Sandopolis Zone

Sandopolis Zone is the 3rd zone of Sonic & Knuckles. It follows Flying Battery Zone and precedes Lava Reef Zone. It is an extremely large desert.

Act One
The first act takes place outside, in a sweltering desert area patrolled by Sandworms, Skorps and Rock'n badniks. Little sand-falls should allow Sonic or Knuckles to progress up them by jumping, while others will automatically be run up. The loops here are different, because when the player goes through one, they circle for a few moments before being launched high into the air. Upon reaching the end of the act, a pyramid rises out of the ground and a Stone Guardian (resembling the Egg Golem from Sonic Adventure 2: Battle) emerges to fight the player. The Guardian can easily be broken apart by a Spin Attack to the head, but will quickly reform and continue to attack. To defeat it, the player must lead it to the quicksand pit on the far left of the area and have it jump in. After beating this mini-boss, the player enters the pyramid.

Act Two
The second act takes place inside the pyramid. Chains hanging from the ceiling throughout the level act as light switches, and the lights must be on or ghosts will appear to attack the player. Switches can be pushed to open doors to subsequent rooms, although they close fast and the player must pass through them quickly. Some rooms have bricks sealed in the walls that release sand flows when broken, which raises the ground level in the room, allowing progression to higher floors but also causing a risk of crushing the player against the roof.

Boss
Doctor Robotnik appears in a giant statue, though he cannot be seen because the head area of the statue, where he sits in his Eggmobile. This is the only vulnerable part, which must be hit twice in order to actually hit the boss. All other areas of the boss are covered with spikes, except for the back of the statue's hand. As the statue proceeds forward, its left hand will move to the front. The player can stand on the appendage and jump from there to attack.

Sonic the Comic
The Sandopolis Zone also appeared in Sonic the Comic's "Mystery of the Sandopolis Zone", a fairly faithful adaptation of the zone's story from Sonic the Comic #47 and #48.

History
Sonic and Knuckles met each other in the Sandopolis Zone while they were independently traveling to the Launch Base Zone (conflated with the Lava Reef Zone here, as it is on a volcano) to prevent the launch of the Death Egg. However, they were sidetracked by the sudden appearance of Captain Plunder, who had traveled to the island with his sky pirates to loot the treasure of the Lost Pyramid of Sandopolis (much to Knuckles' consternation). Along with his assistants Filch and the Professor, he managed to raise the pyramid out of the ground.

A stone Guardian appeared from the pyramid, claiming that all heretics must die. Knuckles and Sonic defeat it by (as in the game) luring it into a quicksand pit, only for the pirates to sneak into the pyramid while they are occupied. The pirates activate an ancient lighting system and load up on treasure from the tomb, but flee for the exits as soon as the lights go out. Knuckles is happy to let them go, since he knows about the pyramid's curse... sure enough, the pirates are soon attacked by three ghosts, and forced to drop their treasure in order to escape.

Still in the pyramid's central tomb, Knuckles breaks an ancient seal on the wall, allowing sands to flow into the pyramid, burying it again. Before they are buried themselves, Knuckles uses his knowledge of ancient texts to lead Sonic into a secret tunnel, by which they escape.

The Pyramid was later unearthed once again by Nack the Weasel, who found it by shrinking the sand with his shrinking technology. Once inside, Nack discovered yet another stone guardian, dedicated to protecting the treasure chamber. By unknown means, he was able to reprogram the guardian to work for him. When Knuckles and Chaotix followed Nack to the Pyramid, they had to fight both the guardian and the pyramid's ghosts. Nevertheless, Nack was able to escape the Pyramid, shrinking down all of the treasure from the Pyramid so that he could carry it away in a single box, then enlarge it later before selling it on. Outside the Pyramid, Knuckles was able to stop Nack from leaving the Island by using Nack's shrink ray to re-enlarge all of the treasure, exploding Nack's plane from within. Nack was captured, but Chaotix left the scene leaving Knuckles to pick up all the treasure, which had been scattered across the desert.

Trivia

 * The entrance to Lava Reef from this zone is different between Sonic and Knuckles. Sonic goes into a tunnel and falls through a hole, but Knuckles runs right over it, even though the pre-released ghosts should indicate that Sonic has been there already.
 * In the first act, there are a couple pools of quicksand that, instead of killing the character, will drag the character down into another part of the level, usually revealing hidden items such as ring boxes. It is hard, however, to recognize the different quicksand pools since they all look the same (although ground below the pool can occasionally be seen) and they all behave in the same way. It is usually best to avoid sinking in any quicksand. The second act has only one of these non-deadly quicksand pools.
 * If the player goes to Act 2 in story mode, the act will be dark. However, if the player uses level select to enter Act 2, the act will start with light.