Submerged Superstructure is a late-game region exclusive to Downpour. It is accessible to every Slugcat except Spearmaster and Artificer. It harbors various remnants of Looks to the Moon's superstructure and dark, submerged caverns filled with aquatic life.
Submerged Superstructure is the remains of Looks to the Moon's superstructure, submerged after the damage her can suffered in its legs after Five Pebbles' first attempt to expel the Rot. While it is most easily explored by Rivulet, most Slugcats can enter and explore the region if they have enough Bubble Weed.
The region is generally composed of deeply submerged, dimly lit spaces. The remains of the superstructure flourish with varied plant life as well as aquatic creatures such as Giant Jellyfish, Jetfish, and Eel Lizards. Leviathans are present in the deeper sections of the region. Glow Weed appear in large quantities in the lowermost water-filled sections.
In Rivulet's campaign, a Rarefaction Cell is to be inserted in the central subregion The Heart. After this, the middle level of Submerged Superstructure and the Looks to the Moon subregion of Shoreline periodically experience functional recovery and gravity reduction.
SPOILER WARNING: Saint This section contains major plot details from Saint. If you have not made significant progress or completed the game as Saint, then read no further!
In Saint's campaign, a few rooms have been changed to make getting to The Heart easier, and some power and gravity has been restored. Saint has a significantly easier time navigating Submerged Superstructure than other Slugcats (besides Rivulet) as their Ascension flight ability allows them to travel faster than Rivulet through water, without using up any breath.
Saint can find an additional Echo inside of Bitter Aerie. This Echo is not required for the Pilgrim passage, nor is it required to complete Saint's campaign, and is difficult to get to without having already completed Saint's pilgrimage.
It is, however, possible to reach the Echo without going through the Heart or completing Saint's pilgrimage. This route through Looks to the Moon's Memory Conflux is probably unintended, as the surfaces in a certain room are technically unable to be grappled onto by Saint, but jumping even on the few frames their tongue makes contact with the surface gives Saint enough upwards momentum to scale the wall. Another option is to use the Squidcadas that spawn nearby.
Vents comprises the lower half of the region, the underbelly of Looks to the Moon's superstructure that is well below sea level. It is deeply eroded and flooded, with a flourishing aquatic ecosystem, mainly populated by Jetfish, Aquapedes, and Giant Jellyfish. These lower passages have been transformed into something comparable to an underwater cave; a series of intertwined curving tunnels that have been narrowed and shaped by a barnacle-like material, coating almost every surface with a thick rough coral layer. The majority of the rooms are fully underwater and mainly illuminated by Glow Weed, with only a handful of air pockets and breathable rooms in which an abundance of Bubble Weed and Bubble Fruit can be found.
The Heart
The Heart is the last stretch of the flooded underground portion of Submerged Superstructure, featuring a wide flooded conduit that leads into a central chamber in which the Rarefaction Cell is to be placed. The conduit has up to 7 connections to other parts of the region, and contains sporadic branch rooms to restore breath. Relocating the Rarefaction Cell restores a fraction of power to Looks to the Moon's facility and causes the core to fill with water, forcing Rivulet to exit or drown. The upper exit cannot be used until this is done. On leaving the core, Rivulet is transported by a strong current up the shaft in The Heart through a one-way passage into Bitter Aerie.
Entering the core reactor without the Rarefaction Cell traps Slugcat, forcing them to quit to the menu or wait until the rain kills them. Entering the core as a Slugcat other than Rivulet or Saint triggers the rain. Entering as Saint kills them.
Powerful currents periodically arise within The Heart after the Rarefaction Cell is inserted, propelling Slugcat towards certain locations and making navigation difficult.
Bitter Aerie
Bitter Aerie is a cold expanse at the top of the region that Rivulet is expelled to after placing the Rarefaction Cell. The lower area contains partially-flooded tunnels and pipes, while the upper area contains a series of ascents and descents through steep cliffsides intermixed with wide surface areas exposed to the snow. In the west, these paths converge to lead to a one-way Karma Gate, the only exit from this subregion, containing sparse Scavenger life in the preceding mountains and passages underneath the surface. While in this subregion, Passaging is disabled.
Similar to The Wall, this area is high enough up that the rain is non-lethal.
An isolated high-altitude tower that can be reached through the right uppermost pathway in Bitter Aerie, whose snowy skies are home to Miros Vultures. This area is one of the locations featured within the ending sequence for Rivulet's campaign, and houses one of Saint's exclusive Echoes.
Like in Bitter Aerie, the rain here is non-lethal.
Submerged Superstructure has the acronym MS in game files, which stands for "Moon Superstructure".
The Vents have many of its room exits hidden for non-Rivulet Slugcats. The exits that are shown highlight the easiest path through the subregion.
In room MS_AI, green electrical discharges can be found, similar to the those found in The Rot.
Entering Bitter Aerie through the use of a warp mod as any character but Rivulet or Saint reveals that there is no snow there, as it is merely a shading effect that is disabled in other campaigns.
Additionally, attempting to leave Bitter Aerie as Spearmaster or Artificer softlocks the game, as it attempts to load a location that does not exist for them.
A muffled version of MOON_SIREN plays when the rain begins to approach, the same audio file that plays in the pre-collapse Looks to the Moon and Waterfront Facility.
Submerged Superstructure is one of the largest regions in Downpour at 134 rooms.
For Slugcats other than Rivulet and Saint, one of the Vulture spawns is guaranteed to be albino.
Downpour. It is accessible to every Slugcat except 
Spearmaster and 
Artificer. It harbors various remnants of 
Looks to the Moon's superstructure and dark, submerged caverns filled with aquatic life.
Five Pebbles' first attempt to expel the Rot. While it is most easily explored by 
Rivulet, most Slugcats can enter and explore the region if they have enough 
Bubble Weed.
Giant Jellyfish, 
Jetfish, and 
Eel Lizards. 
Leviathans are present in the deeper sections of the region. 
Glow Weed appear in large quantities in the lowermost water-filled sections.
Rarefaction Cell is to be inserted in the central subregion 
The Heart. After this, the middle level of Submerged Superstructure and the 
Looks to the Moon subregion of Shoreline periodically experience functional recovery and gravity reduction.
SPOILER WARNING: Saint
Saint's campaign, a few rooms have been changed to make getting to The Heart easier, and some power and gravity has been restored. Saint has a significantly easier time navigating Submerged Superstructure than other Slugcats (besides Rivulet) as their Ascension flight ability allows them to travel faster than Rivulet through water, without using up any breath.
Echo inside of Bitter Aerie. This Echo is not required for the Pilgrim passage, nor is it required to complete Saint's campaign, and is difficult to get to without having already completed Saint's pilgrimage.
Squidcadas that spawn nearby.
All
Monk
Survivor
Hunter
Gourmand
Light Yellow Pearl
Cloak (pre-
Rivulet only)
Aquapedes, and 
Bubble Fruit can be found.
Scavenger life in the preceding mountains and passages underneath the surface. While in this subregion, Passaging is disabled.
The Wall, this area is high enough up that the rain is non-lethal.
Miros Vultures. This area is one of the locations featured within the ending sequence for Rivulet's campaign, and houses one of 
Maps
Creatures
Slugcats
Regions
The Rot.
Spearmaster or 
Artificer softlocks the game, as it attempts to load a location that does not exist for them.
Looks to the Moon and 
Waterfront Facility.
Saint, one of the 
Vulture spawns is guaranteed to be albino.
