Aquatic Base

Aquatic Base is an action stage in Sonic the Hedgehog (2006). The area is a high-tech Soleanna research facility, hidden beneath the waters of Kingdom Valley. It is here that the Duke of Soleanna was conducting time-travel experiments on the Flame of Hope.

Aquatic Base was partially destroyed in the explosion that curtailed the Solaris Project (the event which also created Iblis and Mephiles). Along with the royal palace in Kingdom Valley, it was subsequently abandoned. Some years later, Doctor Eggman clandestinely moved into the facility, using it as a secret base and hangar for his Egg Carrier.

Sonic
This is Sonic's last stage in his story. He goes through here with Tails and Knuckles in an attempt to reach Princess Elise and save her from Eggman once and for all. Sonic uses his trademark homing attack and useful bounce move to reach high ledges and make it across the terrain. There are small globes that Sonic can knock into enemies floating throughout the level. There are also much larger ones that he can ride on to get across deep chasms located in the abandoned base.

Tails
Tails is playable in the last stage of Sonic's story during a section after Sonic's first leg in the base. Tails flies through an area and drops ring bombs on robots that get in his way in order to reach a room with a switch inside. Once there, he must activate the switch so that a doorway will open and Sonic can continue on.

Knuckles
Knuckles is playable in the final stage of Sonic's story. His part comes near the end and is very short. He glides across a deep chasm and activates blue pillar switches by diving into the ground. The shock-wave activates blue pillars and destroys the robots in the area. He then continues to do this until all the robots and switches are turned on and Sonic is given a clear way to continue onward.

Shadow
About halfway through Shadow's story, the player returns to 10 years in the past in order to go through this stage. The player starts off as Shadow during the first leg and switch places with Silver during the second leg. Once Silver activates a switch, Shadow is playable once again for the final leg. Shadow is able to go through by using homing attacks and riding on the optional vehicles provided throughout the stage, such as a G.U.N motorcycle.

Silver
About halfway through Silver's story, the player returns to 10 years in the past in order to go through this stage. The player starts off as Silver during the first leg and switch places with Shadow during the second leg. Once Shadow activates a switch, Silver is playable once again for the final leg. Silver is able to use his psychokinesis to launch the floating globes in the area at the monsters that attack him, as well as use the monsters themselves as projectiles.

Sonic's story
The Aquatic Base is the final level for Sonic in his story. After going back through a rift in time, opened when he and Silver collided two Chaos Emeralds together, Sonic goes through the abandoned factory in order to reach the end and land on Eggman's aircraft, which is about to take off. For unexplained reasons, Tails and Knuckles are also present and are playable during certain parts of the level.

Shadow's story
The Aquatic Base level takes place in Shadow's story at the same time as Silver's. After a fight between the two hedgehogs ended with Shadow being the victor, a rift in time was opened up with Chaos Control. It was a portal that sent them into the past. Shadow and Silver, both determined to find answers, jumped into the portal and reached the Soleanna laboratory to witness the Duke of Soleanna and his scientists as they experimented on Solaris. Unfortunately, the project was a failure and an explosion took place before they could reach the area. Upon inspecting the wreckage, they saw Solaris split apart into the original forms of Iblis and Mephiles. Silver was set to pursue Iblis, while Shadow, after being handed the Scepter of Darkness from a barely alive Duke, set his sights on Mephiles. The player proceeds through the level and is able to play as Silver at one point.

Silver's story
The Aquatic base level takes place in Silver's story at the same time as Shadow's. After a fight between the two hedgehogs ended with Shadow being the victor, a rift in time was opened up with Chaos Control. It was a portal that sent them into the past. Shadow and Silver, both determined to find answers, jumped into the portal and reached the Soleanna laboratory to witness the Duke of Soleanna and his scientists as they experimented on Solaris. Unfortunately, the project was a failure and an explosion took place before they could reach the area. Upon inspecting the wreckage, they saw Solaris split apart into the original forms of Iblis and Mephiles. Silver was set to pursue Iblis, while Shadow, after being handed the Scepter of Darkness from a barely alive Duke, set his sights on Mephiles. The player proceeds through the level and is able to play as Shadow at one point.

Trivia

 * There is no clear explanation, but during the course of Shadow's and Silver's levels, the enemies that are fought are all monsters from the far future. It is possible that both Mephiles and Iblis spawned these enemies, even in their base forms.
 * It should also be noted that this is the first level to feature Mephiles' and Iblis' minions.
 * In the Aquatic Base, the Solaris symbol on the security glass doors is similar to the G.U.N. symbol.
 * The music played during this level is very similar to that of the Lost Impact level in Shadow the Hedgehog.
 * Additionally, Lost Impact takes place in the past (or a memory section from the past in Lost Impact's case) and Aquatic Base is also encountered in the past (at least for Shadow and Silver's part).
 * Aquatic Base and Radical Train are only levels that do not appear in the End of the World stage.
 * Aquatic Base's Act 1 theme sounds similar to rapper Rick Ross' rap song "Diced Pineapples."