|Game||Sin & Punishment: Star Successor|
|Stage #||Stage 2|
- "This massive rollercoaster of a tunnel is surrounded by sea life. The rich ecosystem may be a stunning sight to behold, but there won't be much time to admire it with Moray Keepers awaiting at every turn. A final descent into the trenches reveals even greater challenges."
- — Sin & Punishment official website
Undersea Tunnel is the second major stage in Sin & Punishment: Star Successor.
- "And you're sure there are no enemies down here?"
- — Kachi
- "Positive! Why would enemies hang out in a water tunnel?"
- — Isa
Following the defeat of the Snapper Keeper in Fukuoka, Isa managed to find a route off of Earth 4: after traveling through a water tunnel, he and Kachi could infiltrate a Sky Fortress and hijack a ship stationed there. Thinking they have found a safe, quick route off the planet, the pair make their way to the water tunnel, only to discover that, to their surprise, the tunnel is filled with enemies. Hostile Keepers and Earth 5 soldiers infest the tunnel, and along their way, Isa and Kachi must fight these enemies off. After defeating a massive defense system known as Brimstone, Kachi continues to question Isa about humanity.
Their conversation is interrupted, however, by Armon Ritter, a member of the Nebulox. After they manage to defeat him, Armon acknowledges his defeat and admits that he underestimated them. He tells Isa that the Nebulox has set up a base at Mt. Fuji, instructing him to bring Kachi's corpse there when he gives up on fighting the Nebulox. Isa and Kachi disregard this, opting to escape Earth 4 rather than pursue the Nebulox. They head towards the Sky Fortress, hoping to procure a ship.
The Underwater Tunnel serves as Star Successor's take on the video game archetype of the water-based level. However, while many water-based levels in various games have been met with severe criticism and widespread hatred, the Underwater tunnel has, like all stages in the game, been met with great praise. Taking place in a water tunnel, the stage barely makes use of walking and jumping mechanics at all, and instead, almost all of the stage is spent using the characters' levitation devices. The protagonists are often surrounded by marine foes on all sides, many of which are actually not hostile, and merely serve to give the player more points. The Earth 5 military has taken on marine technology in order to fight Isa and Kachi; soldiers on what appear to be jet-propelled surfboards will surf around the inside of the tunnel while attacking, and the protagonists will often find themselves under attack by boats and underwater vessels.
At certain points in the level, the protagonist will go around a corner too fast, and will be hurled out of the tunnel into the sea for several seconds. This proves helpful, as enemies will not see the player when he or she is outside of the tunnel, giving him or her a chance for a sneak attack. While the soldiers inside the tunnel cannot be seen, sprays of water can be seen on the outside of the tunnel from where their surfboards are in contact with the wall of the tunnel.
After defeating the Medusa Keeper, the water tunnel will turn straight downward, and the player will begin a slow descent into the ocean depths. Many enemies will now provide air bubbles, which, for some reason, can serve as explosive projectiles that can be deflected with melee attacks. This comes in handy against stronger enemies and the Moray Keeper. After defeating the aforementioned boss, the player will soon face the NS Barracuda, followed by Brimstone, and, finally, Armon Ritter himself.