The Aquatic Mine is an abandoned coal mine located within 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 absolutely 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, adjustment being necessary to reach the higher levels of the forlorn place. There is an optional Level Up Item available for Knuckles in this level, the Air Necklace. Consider it an infinite oxygen tank that allows Knuckles 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.
Find the lost chao
From the beginning go up the center structure to reach the top floor. Inside here, on the side where you find the 10 ring capsule and the computer, you should find an ancient shrine behind a pile of wood. Play the Mystic Melody on it and you should see a door open. Go inside and swim down. This will take you to the room where the ghosts are. Find the two skulls statues in the corner, then look for a shadow on the ground, or rings up higher. Once you find these, take them up. This will take you to a tunnel. Go to the end of the tunnel (there is a extra life behind the spiked ball) and take the pulley up. You should see the Chao!
Collect the Emerald pieces in 3 minutes 0 seconds!
This mission is the same as the first, except with a time limit. Work as quick as you can to find the missing pieces. The quicker you are, the better grade you will get!
Collect 100 rings!
For some quick rings, collect all the ones around you, then go up to the top floor and hit the ten ring capsule in there. The rest of the rings can be found scattered through out the board. Go to the route leading to the Air Necklace, as there are quite a few rings there, including two twenty-ring capsules!
Clear Hard Mode
Head for the large wheel in the northeast corner of the main room. 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, dive into the mineshaft along the western wall on the second floor and navigate the tunnels. There is only one route, so the player much keep going until they find the first Emerald Shard at the dead end. Be careful of the final descent, which is littered with spiked balls.
The final Emerald Shard is near where the lost Chao was, but instead of taking the pulley, look behind the spiked ball.
Big the Cat sighting
To the left of the Wide Spring, there is a mineshaft which has Big inside it. However, Big only appears inside the mineshaft when the water level is set to the first or second level.
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. Whereas 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."