The Air Necklace consists of a grey necklace-styled oxygen bottle shaped like a handle. It allows Knuckles to stay underwater without drowning, eliminating the need to search for air bubbles or return to the surface to breathe. This Level Up Item is extremely useful (though not required) during Knuckles' section of the Cannon's Core, which consists of very deep submerged passageways.
The Air Necklace Level Up Item is found in the Aquatic Mine, and is only accessible when the water level is at its minimum. Even after finding the entrance to its hiding spot, it is hard to reach since the passageway leading to it is completely submerged, maze-like, and ridden with spikes. It is also very dangerous to reach because there are no air pockets or bubbles until the end where the Air Necklace is located, greatly increasing the chances of the player drowning.
To reach the Air Necklace, the water level in the Aquatic Mine must first be reduced to its lowest level. This can be done by heading up to the floor above the main room, which is done by taking the pulley found on the top on the central square structure in the main room. Once there, the player must touch the switch found in front of the sign that says "3" and the water level should be lowered.
With the water level lowered, the player must head into the underwater brick room by going into the shaft found in the corner of the main room, where they will see the octagonal tunnel. Once the player sees it, they must dive underwater. Once at the bottom of the brick room, they must then head into the tunnel with the Dash Panel which will lead to a room with several wooden 2-by-4s. From there, the player must dive to the bottom on the left of the tunnel in this room, and they will find themselves in another brick room. They then need to head then into the tunnel found in the brick room and will then arrive in a vertical tunnel. The Air Necklace can then be found on the top of this vertical tunnel in a dry room, which can be reached by surfacing from the tunnel. The player should try to do this quickly to avoid running out of air.
After the multiverse was rebooted following the events of Worlds Collide, the Air Necklace became a piece of equipment used by GUN agents.
This is the only one of Knuckles' Level Up Items in Sonic Adventure 2 and Sonic Adventure 2: Battle that does not have an equivalent with one of Rouge's Level Up Items.
Omochao never states that the Air Necklace is exclusive to Knuckles when mentioned in the Tutorial Section, like he does when explaining Magic Hands and Protective Armor. The only other exclusive Level Up Item to not be mentioned as such by Omochao is the Bounce Bracelet.