Aquatic Mine

Aquatic Mine (水生鉱山 suisei kōzan) is the eighth stage in the Hero Story of Sonic Adventure 2 and Sonic Adventure 2: Battle. This stage is only playable as Knuckles. The level's theme song is "Dive into the Mellow".

Overview
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 10 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 20 ring capsules!

Archie Comics
The Aquatic Mine has also appeared in the Sonic the Hedgehog and its spin-off comic series' published by Archie Comics. It first debut in Sonic Universe #64, and it technially called "Aquatic Mine Zone". It appears in the Post-Super Genesis Wave timeline.

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 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.

Trivia

 * 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."
 * The 2P stage Pool Quest is part of this stage.
 * On a lower level on the stage, a hint monitor says "The ghosts are waiting for you!" This may have been intended to warn the player about the Boos that were hiding and not the ones on show.