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 Solaris during the Solaris Project.

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, Dr. Eggman clandestinely moved into the facility, using it as a secret base and hangar for his Egg Carrier.

Sonic
Now that Sonic the Hedgehog has rewritten the future and saved Princess Elise from perishing in the crash, he can set about accomplishing his ﬁnal showdown with Dr. Eggman. First, Sonic must slip beneath the waves and inﬁltrate the old Aquatic Base, an undersea laboratory used by Elise’s father before the tragic accident that ended his life. Eggman has stocked the base with his robot army and uses it for experiments. With the help of Knuckles and Tails, Sonic must clear out the mechanized forces and reach Dr. Eggman before he can escape in his Egg Wyvern.

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
Shadow the Hedgehog slips under the waves to inﬁltrate the Aquatic Base, an underwater science complex appropriated by Dr. Eggman. The site was abandoned after a tragic accident with the Duke of Soleanna, but now its steely corridors are buzzing with activity. Iblis’s ﬁery minions have infested the base and are causing Shadow considerable trouble as he presses deeper into the complex to hunt Dr. Eggman. Shadow is not alone in this endeavor, though. Silver the Hedgehog is also exploring the Aquatic Base in search of the Iblis Trigger the being that sets off a sequence of events that leads to Silver’s charred and ruined future.

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
Deep beneath the waves of Soleanna, the Aquatic Base served as a secret laboratory for the Duke, but after his fatal accident, the facility was abandoned. Recently uncovered by Dr. Eggman, the lab is now one of his bases of operation. However, when Silver the Hedgehog dives deep to inﬁltrate the Aquatic Base, he’s confronted not by metal monsters, but ﬁery beasts like those he challenged in Crisis City. The minions of Iblis have caught up to Silver, and if he wants to edge closer to the truth and the solution for undoing Iblis’s ﬂame ravaged future Silver must extinguish the monsters and escape the base. Fortunately, he receives a little help from Shadow The Hedgehog.

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
Aquatic Base is the final level for Sonic in his story. After going back through a rift in time which was opened when he and Silver induced Chaos Control simultaneously with two Chaos Emeralds together, Sonic goes through the abandoned factory in order to reach the end and land on Eggman's Egg Carrier, which is about to take off. For unexplained reasons, Tails and Knuckles are also present and are playable during certain parts of the level. The level is filled by various of Eggman's robots except Egg Keeper making it the hardest level in Sonic's Story. However this is the easiest level to get an "S" Rank by destroying robots.

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. The Stage is filled by various of Iblis and Mephiles Minions to prevent Shadow from sealing Mephiles.

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. Like Shadow this version of the stage is filled by various of Iblis and Mephiles Minions to prevent Silver from sealing Iblis.

Trivia

 * There is no clear explanation, but during the course of Shadow's and Silver's Action Stages, 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 Action Stage 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 Action Stage 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).
 * This Action Stage and Radical Train are the only ones that do not appear in the End of the World stage.
 * It is possible after Shadow taking the control of the level to complete some of Silver's sections by doing Shadow's wall Glitch in one of the doors in the room before going to silver after defeating Iblis Minions.