The Aquatic Mine is an abandoned coal mine located in the vicinity of Pumpkin Hill. Knuckles discovers this place in his search for Master Emerald shards. In keeping with the imagery developed in his previous stage, many of the symbols found about the mine seem very Halloween themed. This level is also infested with Boos, enough to the point that a warning sign has been posted. It appears that at some point during its operation, the mine flooded. There are three switches present in the level that affect the height of the water within the mine, with adjustments being necessary to reach the higher levels of the forlorn place.
Level Up Item
There is an optional Level Up Item available for Knuckles in this level called the Air Necklace, which allows him to stay submerged for any period of time without drowning. When returning to this level to complete further missions/goals, this Level Up Item is very helpful.
Collect 100 Rings
For some quick rings, the player should collect all the ones around them at the start of the level, then go up to the top floor and hit the ten ring capsule. The rest of the rings can be found scattered throughout the level, with a notable amount placed on the path leading to the Air Necklace.
Find the lost Chao
From the beginning of the stage, the player should go up the center structure to reach the top floor and keep the water level at one. On the side where a 10 ring capsule and a computer can be found, the player will find an ancient shrine behind a pile of wood. They must play the Mystic Melody on it, and they will see a door open. The player should go inside and swim down, which will take them to the room where the ghosts are. They must find the two skulls statues in the corner, then look for a shadow on the ground, or rings up higher. Once the player finds these, they should take them up, which will lead them to a tunnel. The player must go to the end of the tunnel (there is an extra life behind the iron ball) and take the pulley up, and the Chao can be found just ahead.
Collect the missing Emerald pieces in 3:00
This mission is the same as the first, except with a time limit. The player must work quickly to find the missing pieces.
Clear Hard mode
To start, the player should head for the large wheel in the northeast corner of the main room, then stand on the edge of the southern plank and put the Sunglasses on. A spring should appear beneath the player, which can launch them to the second Emerald Shard. With the water level at level one, they must dive into the mineshaft along the western wall on the second floor and navigate the tunnels. There is only one route, so the player must keep going until they find the first Emerald Shard at the dead end. They should be careful of the final descent, which is littered with iron balls.
The final Emerald Shard is near where the lost Chao was. Instead of taking the pulley, the player should look behind the iron ball.
Go to the top floor and hit the third Switch. Then, from the starting point, enter the mineshaft at the southern wall on the bottom floor, and the second Chao Container can be found near there.
While the third Switch is activated, stay on the top floor while the water gets sucked up. Go then past a caution sign with ghosts on it, and keep going straight to find the third Chao Container on a ledge.
In Sonic Universe #64, Knuckles and the Chaotix went to the Aquatic Mine Zone after encountering the Hooligans, where both parties were present in the mine to search for a Chaos Emerald located in this zone. While the Hooligans had trouble finding the emerald due to confusing pathways, Knuckles and the Chaotix were having trouble dealing with themselves, being disturbed by the thought of ghosts and the fear that the Hooligans have found the emerald. However, Knuckles and the Chaotix were able to find the Emerald in Chip's possession.
There is apparently a connection between the Aquatic Mine and the sewers of Central City, as Knuckles emerges from a manhole in his next cut-scene, simply stating to his shocked friends that he had "gotten lost in the mines."