Underwater theme

Knuckles underwater in Aquatic Ruin Zone.

Stages with an underwater theme have significant sections set underwater. Enemies tend to be modeled on fish or other aquatic creatures, but the main obstacle in underwater levels is the water itself. Since Sonic cannot swim or float, he runs along the bottom of the levels just like on land, except that he moves slower and must regularly replenish his air supply or else die by drowning. Air can be obtained by surfacing or by swallowing air bubbles. Partly for this reason, underwater levels are often the most difficult levels in a Sonic game.

Other common elements in underwater levels include water spouts that rocket Sonic to new locations, fast-moving flows of water that Sonic must either ride or resist, and rising water levels that Sonic must keep ahead of as a sort of time limit to certain sections.

The underwater theme is one of the most versatile in terms of setting and can overlap with many other stage themes, including the ancient ruins theme, industrial theme and winter theme.

Note that the fluid making up the level need not strictly be water. Chemical Plant Zone's pink Mega Mack has exactly the same properties.

Stages with an underwater theme

• Labyrinth Zone, Sonic the Hedgehog (1991) - The first and arguably most famous example of the underwater theme, Labyrinth Zone (set in a submerged ruin) introduced most of the elements that came to define the theme.
• Scrap Brain Zone - Sonic the Hedgehog (1991) - Act 3 only; it also incorporates elements of the ancient ruins theme since the level is a different color-pallete of Labyrinth Zone.
• Aqua Lake Zone, Sonic the Hedgehog 2 (8-bit) (1992)
• Chemical Plant Zone, Sonic the Hedgehog 2 (1992) - A primarily industrial level, Chemical Plant Zone is flooded with pink Mega Mack, which functions exactly like water.
• Aquatic Ruin Zone, Sonic the Hedgehog 2 (1992) - This stage also incorporates elements of the ancient ruins theme.
• Tidal Tempest, Sonic the Hedgehog CD (1993) - Past, present and bad future timelines incorporate an ancient ruins theme similar to Labyrinth Zone while either future timeline incorporates an industrial theme.
• Aqua Planet Zone, Sonic Chaos (1993) - This stage also incorporates elements of the ancient ruins theme.
• Hydrocity Zone, Sonic the Hedgehog 3 (1994)
• Carnival Night Zone, Sonic the Hedgehog 3 (1994) - This zone has an amusement park theme, but also includes underwater areas. This theme is removed when playing as Knuckles.
• IceCap Zone, Sonic the Hedgehog 3 (1994) - Mainly a winter themed stage, this zone contains minimal underwater areas in Act 2.
• Tidal Plant Zone, Sonic the Hedgehog Triple Trouble (1994) - This zone also incorporates elements of the industrial theme.
• Lake Rocky, Tails Adventure (1995) - This stage also incorporates elements of the coastal theme.
• Cavern Island, Tails Adventure (1995) - This stage also incorporates elements of the underground theme.
• Labyrinth of the Sea, Sonic Labyrinth (1995)
• Blue Marine Zone, Sonic Blast (1996) - This stage also incorporates elements of the ancient ruins theme.
• Aquatic Relix Zone, Sonic the Hedgehog Pocket Adventure (1999) - This stage also incorporates elements of the ancient ruins theme.
• Ice Mountain Zone, Sonic Advance (2001) - This zone has underwater areas, but mainly incorporates the winter theme.
• Leaf Forest, Sonic Advance 2 (2002) - Despite its name, Leaf Forest has no elements of a forest theme, although elements of a Green Hill theme are present.
• Ocean Base, Sonic Advance 3 (2004)
• Twinkle Snow, Sonic Advance 3 (2004) - Similar to Ice Mountain Zone, this stage also mainly incorporates elements of the winter theme.
• Water Palace, Sonic Rush (2005) - This stage also incorporates elements of the urban theme.
• Aquatic Base, Sonic the Hedgehog (2006)
• Plant Kingdom, Sonic Rush Adventure (2007) - This stage also incorporates elements of a forest theme and a coastal theme (Act 1 only).
• Coral Cave, Sonic Rush Adventure (2007)
• Pirates' Island, Sonic Rush Adventure (2007) - This stage has underwater areas but mainly incorporates elements of the ancient ruins theme.
• Lost Labyrinth Zone, Sonic the Hedgehog 4: Episode I (2010) - Act 3 only; mainly incorporates elements of the ancient ruins theme.
• Dolphin Resort, Sonic Free Riders (2010) - This course has an underwater segment, but mainly incorporates elements of the coastal theme.
• Aquarium Park, Sonic Colors (2010)
• Chemical Plant, Sonic Generations (2011) - This stage mainly incorporates elements of the industrial theme.
• Seaside Hill, Sonic Generations (2011) - The underwater theme is featured in Act 1, but the coastal theme is also used in Act 2.
• Sylvania Castle Zone, Sonic the Hedgehog 4: Episode II (2012) - This zone is a hybrid of ancient ruins theme and the underwater theme. The underwater theme predominates mainly in the second act.
• White Park Zone, Sonic the Hedgehog 4: Episode II (2012) - This zone is a hybrid of the amusement park theme, the winter theme and the underwater theme. The underwater theme predominates mainly in the third act.
• Burning Depths, Sonic & All-Stars Racing Transformed (2012)
• Tropical Coast, Sonic Lost World (2013) - This stage mainly incorporates elements of the coastal theme.
• Hidden Palace Zone, Sonic the Hedgehog 2 (2013 re-release only) - This stage mainly incorporates elements of the ancient ruins and underground theme.
• Undersea Bolt, Sonic Boom: Rise of Lyric (2014) - This level also incorporates elements of the ancient ruins theme.