A time in the future where, for whatever reason, the Earth has become entirely or partially flooded by rising seas.
In modern fiction, this often takes the form of a Green Aesop about Climate Change, as potentially catastrophic rise in sea levels are a very real danger of greenhouse conditions melting the polar ice caps and sending their meltwater into the ocean. This kind of setting can be brought about by other reasons, although most involve some kind of natural or unnatural disaster. In more fantastical settings, this may occur in the wake of The Great Flood if the waters never recede.
The severity of the flooding often varies between works. More downplayed examples restrict the flooding to coastal areas and plains; these may become great drowned morasses or be entirely covered by wide shallow seas, leaving highlands and mountains above the waves, although usually still humid and swampy. In more extreme examples the entire planet can become covered by a pole-to-pole ocean, occasionally with a few islands poking through to mark the peaks of mountain ranges. However, works employing this angle tend to exaggerate the extent to which this can occur — if all the ice on the planet were to melt, this would "only" release enough water to flood most coastal plains and lowlands; covering all continental masses would require a lot more water than exists on Earth today, liquid or frozen alike, and while there have been times in the distant past when Earth's ice caps melted completely — most of the Mesozoic, for example — there was still plenty of dry land.
In more downplayed settings, humanity may be able to scrape along in the drowned lowlands, sometimes inhabiting the higher reaches of buildings or natural formations poking above the waterline, or may retreat altogether to higher ground. In the most intense cases of global flooding, survivors will often be forced to live on rafts, ships, and other waterborne vehicles drifting along the currents, although luckier sorts may be able to hold on in a City on the Water or an Underwater City. Either way, humanity will likely develop into highly nautical societies. Survivors will typically have to deal with intense resource scarcity, since all the stone and metal and forests are at the bottom of the sea, and remaining artifacts and technology will be rare and precious commodities. This often leads to a full-on Scavenger World if they come to rely on gathering flotsam or diving to submerged ruins for artifacts and materials. In cases where islands or other emerged lands exist, they'll either be the main refuge of society or semi-mythical promised lands the seabound survivors yearn to find.
A Sunken City or several is a common visual element. Depending on the severity of the flooding, these may be present as mazes of half-drowned buildings along the coasts, sometimes draped in plant life or inhabited by Disaster Scavengers, or as fully submerged ruins on the ocean floor.
Subtrope of After the End, Climate Change, and Single-Biome Planet. In regards to Apocalypse How, these events are of planetary scale but their level of disruption varies between works. Remaining civilization may be prone to Ocean Punk.
- Agent Aika: In the future, disastrous earthquakes have caused most of the Earth's land to sink under the sea. As this has left a great many cities and treasures underwater, divers known as Salvagers make a business out of diving to the bottom of the ocean to recover these things.
- Blue Submarine No. 6 takes place after the apocalyptic flooding of Earth by Emperor Scientist Dr. Zorndyke, with the few remaining cities ravaged by his army of half-human Fish People, his "children" and intended heirs to the new world.
- Daphne in the Brilliant Blue is set hundreds of years after the planet was flooded by global warming, and the only remnants of humanity are the descendants of several underwater cities that resurfaced and colonized the remaining landmasses. Siberia is now a tropical vacation paradise, and virtually all transportation is submersible or seafaring to some degree.
- Ekrano takes place in a future where the Earth is covered in a global ocean infested with sea monsters called Kujirani.
- Future Boy Conan is set after the oceans have flooded the world and the bad guys live on an island that's the sole surviving industrial center on the planet (and even they are forced to scavenge from the ocean floor).
- Gargantia on the Verdurous Planet is set in the distant future, where the Earth has turned into a giant ocean without a single piece of land above water. Humanity survives by pillaging sunken ships and submerged ruins and connecting dozens to hundreds of ships to enormous fleets such as the eponymous Gargantia and coexists with the powerful, intelligent Whale Squids that also inhabit the future oceans.
- La Maison En Petits Cubes by Kunio Kato transpires on an Earth that's mostly seawater. The level rises steadily, as the main character discovers one morning while getting out of bed when his feet get wet in inch-deep water on his floor. The solution is to build another room atop his current room, which he has done five or six times previously. There are a few other people that visit him, always traveling by boat, which is also how the man acquires his building materials.
- Neon Genesis Evangelion: When Adam was awakened in the middle of Antarctica, kicking off the Second Impact, a "ripple" almost a quarter-mile (four-hundred meters) tall in places spread outward, wiping out every coastal city in the southern hemisphere and resulting in a lot of underwater real estate.
- Patlabor: The Humongous Mecha were originally developed to construct barrier dams to protect coastal cities from being flooded due to Climate Change. Downplayed, as the flooding is not catastrophic and is merely part of the background.
- Yokohama Kaidashi Kikou: Due to an environmental shift whose details are largely unrevealed, sea levels have risen dramatically, submerging a great many coastal cities completely.
- Drowntown: Future London has become a City of Canals, thanks to Climate Change. Among other things, the flooding means that water-based transport is now considerably more common — one of the main characters is an aqua-courier, riding through what used to be streets.
- Give Me Liberty: In the future, rising sea levels have turned the streets of New York City into Venice-like water canals.
- The Jetsons: In the comic book adaptation, the reason the family lives in the sky is due to flooding. This isn't the case in the cartoon it was adapted from, where we see the surface and it looks fine.
- Ultron Forever: In the future where Danielle Cage is the new Captain America, New York has been flooded and Dani protects it from pirates and scavengers.
- A.I.: Artificial Intelligence: The backstory has Climate Change destroying Earth's ecosystems and causing sea levels to rise by a hundred meters. Most of the Third World is effectively uninhabitable, while the rich nations managed to use their advanced technology to survive.
- The Day After Tomorrow: A Climate Change catastrophe causes New York City to flood. Then the falling temperature causes the flood waters to freeze over.
- Split Second (1992) takes place in (then-) 20 Minutes into the Future (in 2008), after the melting of the Arctic ice raised water levels around the world. The movie itself is set in a partially-flooded London, large portions of which have had to be abandoned to the rising waters.
- Star Wars: The planet Kamino, home of the cloning facilities of the Republic, was flooded by melting glaciers in the distant past. The Kaminoans survive on cities raised by pillars above the eternally stormy ocean that now covers the planet.
- Waterworld is the aesop version, created when runaway global warming floods the entire planet. Humans mostly inhabit "atolls", ramshackle floating villages built out of whatever junk and flotsam could be scavenged from the sea, but there are also Drifters who spend their entire lives sailing nomadically between villages on one-person boats, aquatic mutants with gills behind their ears, and the Smokers, feared pirates with access to the only remaining motor craft. Food plants and soil have also become rare and valuable luxuries. Myth claims that a single piece of dry land remains, and most of the movie follows a hunt for this island. In the end, it's revealed that the island exists — it's the tip of Mount Everest.
- The World Sinks Except Japan: Climate Change has made the sea levels rise so much that every landmass in the world has sunk... except for Japan.
- Arctic Rising (and its sequel Hurricane Fever) takes place in a warming near future where the Arctic Circle is melting, the Northwest Passage is a major shipping corridor, and Caribbean islands like Anegada sink beneath the waves forever.
- Conciencia y Voluntad takes place 20 Minutes into the Future, where runaway Climate Change has sunk all coastal cities — the walls around La Plata are partly to keep the waters at bay, and partly to provide cover against the roamers of the ruins of sunken Buenos Aires.
- Dark Life: After the ocean has raised and washed away the Earth's oceanfront property, the only land available is on the bottom of the ocean. Determined Homesteaders in underwater farms have to battle pirates, a corrupt government, and in the case of some of the characters Fantastic Racism from being born with superpowers.
- Drowning Towers (or The Sea and the Summer) by George Turner describes a future in which Melbourne is partially submerged in water. As the tops of skyscrapers are above the water level, they are still inhabited by the cities' poorer classes.
- The Expanse: Climate Change has resulted in many coastal buildings getting lost in the water. While those in space are killing each other over water, Earth is practically drowning in it.
- Flood and its sequel Ark center around the transformation of the Earth into an ocean planet due to the release of massive reservoirs of water from within the Earth's mantle. Humanity on Earth survives mostly on massive rafts and ships and in a single underwater city, although in Ark there's talk of genetically engineering an aquatic human species.
- The Maze Runner: In the backstory, a Solar Flare Disaster has melted the ice caps. Mark's flashbacks in The Kill Order feature him living in a skyscraper after the city was flooded with scorching water.
- New York 2140: In the titular year, the Big Applesauce has become a City of Canals not unlike Venice due to Climate Change.
- Star Carrier: Climate Change combined with a couple Colony Drops has caused much of the low-lying east coast of North America to become flooded, to the point where it was necessary to move the capital of the United States of North America to Columbus, Ohio.
- Vorkosigan Saga: The stories are set far enough into the future that sea levels have already risen; London is protected by a system of barrier walls.
- The Windup Girl: This is a major background element and plot point. The novel takes place in 23rd century Bangkok, which is actually below sea level after Climate Change has taken its toll and only survives thanks to enormous sea-walls and powerful pumps that work throughout the monsoon season.
- Star Trek: Deep Space Nine: In "Our Man Bashir", this is the goal of James Bond-esque villain Dr. Noah — flood the Earth and wash away humanity, save for a few survivors at Noah's facility on Mt. Everest who will become a new human race.
- As in ths books Earth in The Expanse has experienced significant sea level change. Several shots of the opening credits show time lapse images of melting glaciers and sea walls being constructed to protected Liberty Island and the rest of New York which also shows up in several epsiodes as part of their establishing shots.
- Cerulean Seas: An unknown event caused a global flood that covered most of the world in water and swallowed the cities of the drylanders, leaving the game's current setting dominated by scattered islands and vast seas.
- Anno Domini: Anno 2070 is set in a post-sea rise Earth to justify the franchise's core gameplay concept of settling remote islands within a futuristic setting.
- Battle Engine Aquila takes place after rising sea levels submerge almost all livable land, leaving only thirteen islands that the survivors war with each other over.
- Brink: 20 Minutes into the Future, global warming and rising sea levels have rendered most of the earth a giant ocean, with the Ark, a City on the Water built before the flooding, and a few mountaintops as the only known places where humans survive. The Ark itself is suffering an Overpopulation Crisis due to holding nine times as many people as it was originally designed to host, leading to resource shortages, severe wealth inequality, resentment between the refugee community and the Ark's Founders, and eventual civil war.
- Call To Power: If enough pollution occurs, the sea level can rise.
- Civilization VI: In the expansion "Gathering Storm", polar ice caps begin to melt and some coastal land tiles become flooded when enough carbon dioxide is released into the atmosphere from industrial buildings and units.
- Final Fantasy Legend III takes place in three different time periods, broadly defined as the Past, Present and Future. Thanks to an entity from the Pureland, the whole world is being flooded; the Future is so flooded that what started as two continents with a couple smaller land masses has been reduced to six small islands.
- Hydrophobia takes place in a future where much of the world is flooded and massive ships now serve as floating cities.
- In the Hunt is set in the distant future, after the powerful D.A.S. terrorist organization unleashed a doomsday device that caused the polar ice caps to melt and flood most major cities in the world. Players assume control of a powerful prototype submarine as they battle through legions and legions of D.A.S. enemies in six underwater-themed levels.
- The Legend of Zelda: The Wind Waker: The Great Sea the game is set in is what remains of the ancient kingdom of Hyrule after it was flooded by the gods in order to protect it from Ganondorf when the Hero of Time did not reappear to save it. The islands of the sea are the highest mountaintops of the ancient kingdom, to which the people of Hyrule were forced to flee as the oceans rose.
- Lethal Skies is set in an Ocean Punk-type setting after melting glaciers have flooded most of the planet.
- Mega Man Legends takes place thousands of years later in the encompassing Mega Man timeline, and the opening narration states that the world is filled with water. What few settlements left around the world exist on islands or are built over the ruins of a past civilizations, if not both. Earlier installments in the Mega Man franchise show much more landmass among the various areas of the world.
- Raft: In the full Steam version, it's revealed that the reason you are Lost at Sea is because of the ice caps melting and submerging most of the world. While there were some attempts to prevent this through reforestation projects, the government thought they cost too much money and spent their funding on floating cities and yachts instead.
- Splatoon: The backstory indicates that the game is set millennia after the extinction of humanity (and most mammals in general) due to global warming rising the sea levels, paving the way for various sea-life to evolve and become the new dominant species on the planet. According to the first game's official art book, another contributing factor was the nuking of Antarctica during World War V, melting most of the continent.
- Submarine Commander: The Earth experienced global warming so fast that the crew of the titular submarine doesn't realize it, and when they surface, it's all sea.
- Submerged takes place after climate change has flooded the world, and is set in the remnants of a metropolis whose tallest buildings are now like an archipelago of small islands.
- Homestuck: Post-Scratch Earth turns into this once the former empress of the trolls takes over and Alterniaforms it into a state more comfortable for an aquatic alien like herself. By the time her flood is done, humans are all but extinct and Earth is completely covered by oceans, broken only by floating prefab slums home to alien exiles and by Dirk's home on top of a ruined skyscraper.
- 17776 uses an Establishing Shot of the Statue of Liberty chest-deep in water by the titular year; a Freeze-Frame Bonus shows major geographical changes, including Florida and Louisiana completely flooded. The epilogue reassures people that affected populations were safely relocated with future technology.
- Bojack Horseman: In "Ruthie", Princess Carolyn's eponymous descendant is shown to live in an underwater city with the rest of civilization after the ice caps melted and the water levels rose. However, Ruthie is Princess Carolyn's coping mechanism after her awful day; she doesn't exist, and it is all a fiction.
- Futurama: In "The Late Philip J. Fry", while travelling through a long series of future eras, the characters come across one where the Earth is covered by water and roamed by ferocious sea monsters.