A waterfall located in the midst of a larger body of water that pours continuously inwards — as if rushing over the lip of a suddenly submerged cup. Logic dictates that water flowing over the falls should fill the basin into which it flows, but through some means a void remains and the falls continue to flow. The physics behind this phenomenon can be handwaved away with environmental magic, or the author can conjure up a high tech, super-efficient water recycling machine that somehow keeps the basin from flooding. The space at the center of these waterfalls seems to be coveted real estate, drawing all sorts of examples of Bizarrchitecture. Can be the mark of a not-quite-so-underwater Underwater City.
Dramatically speaking, there's always the possibility that whatever machinery or magic drains the basin will break and the protagonists will be faced with a rapidly flooding arena. Whether or not they can swim is a different story.
Sister trope to Waterfall into the Abyss, in which water cascades off the side of a Floating Island or the edge of a Flat World while being fed by a seemingly inexhaustible source. Also related to Mega Maelstrom, which is mother nature's version of this but with spinning. Can lead to an Inevitable Waterfall moment, even in the middle of an ocean.
- Dragon Ball Z: Goku created a temporary such hole when his Spirit Bomb hit Freeza.
- One Piece:
- During the Enies Lobby arc, Robin is being held on Enies Lobby, an island hanging in the middle of an abyss by a narrow strip of land far too frail to actually support it. The abyss is in the middle of the ocean, which drains eternally into this bottomless pit.
- Another one shows up much later at the start of the Egghead arc. It is all that remains of the kingdom of Lulusia after being destroyed by a secret World Government aerial superweapon by order of the shadow ruler Imu.
- Breath Waker: When the Cataclysm drowned most of the world beneath the Great Ocean, the Oracle of Time saved her country, Labrynna, by freezing the encroaching tidal wave in time. As a result, modern Labrynna lies at the bottom of an immense hole in the ocean, maintained by the magic of the Oracles who keep it from falling in, and Labrynnans seeking to explore the rest of the world need to bodily climb over the syrupy, time-frozen water of the Seawall to reach the surface of the sea.
- Fantastic Four: Rise of the Silver Surfer: Silver Surfer creates several seemingly bottomless craters at various locations around the Earth to prepare it for Galactus, with one of the craters appearing in the middle of the Thames in London. The water ends up completely draining into the crater while the four are distracted saving the damaged London Eye Ferris Wheel. Stranded boats can be seen on dry riverbed.
- Sonic the Hedgehog 2: The temple containing the Master Emerald is hidden underneath the ocean and accessed through a large hole in the sea ringed by foaming waterfalls and with a bottom obscured by mist.
- The Adversary Cycle: Played for horror in Nightworld where portals to Another Dimension have opened sending nightmare creatures swarming across the Earth. Some of them open in mid-ocean, swallowing ocean liners and creating vast whirlpools during the day and reversing at night into mile-high fountains of water, littering islands with dead fish and... other things.
- Andor: The Narkina 5 prison complexes are a technological version, eight story tall structures with a hollow center built on the lakebed with a rim constructed around the pit that the lake/reservoir drains into. They're constructed this way so the falling water can be exploited for hydroelectricity.
- "Anno Domini": The Anno 2005 default starter Temperate zone has one of these, caused by a series of catastrophically botched hydroelectric dams.
- Assassin's Creed IV: Black Flag: A glitch can cause the water under the ship Jackdaw to render incorrectly or not at all, resulting in a steep-sided hole in the ocean where the ship ought to be.
- Final Fantasy XII: The Ridorana Cataract is a huge abyss at the eastern edge of the sea, ringed by immense waterfalls, that prevents access to the land beyond — ordinary ships get pulled into the cataract, while regular airships lose power if passing over it since the whole area is a jagd.
- Genshin Impact: In the western area of the Inazuma archipelago lies Watasumi Island, which is a large and colorful sinkhole whose people are descendants of an ancient underground civilization who were brought to the surface by their serpent god, Orobashi no Mikoto. While the rest of Inazuma worship the Electro Archon, the people of Watasumi Island despise the Electro Archon for slaying their god, though the Divine Priestess wishes for these old grudges to come to end.
- Grandia II: The Granacliffs are a series of chasms which radiate out from the point where the God Granas struck down the Dark Gold Valmar. When the cliffs pass through the ocean, a linear sea sinkhole occurs.
- Guilty Gear: Japan is wiped off the face of the Earth by the command-type Gear Justice, leaving behind a large hole in the ocean ringed by waters constantly pouring down. Its most notable appearance is as a stage in Xrd.
- Luigi's Mansion: Dark Moon: The Treacherous Mansion miraculously stands on a very precarious rock in the middle of a giant sea sinkhole.
- LEGO Marvel Super Heroes 2: Kang's citadel in Chronopolis is situated in the middle of several spacially and temporally displaced locations, almost all with their own body of water, that constantly flow down waterfalls into an empty void the citadel floats above. The dimensional wackiness of Chronopolis can be used to explain away why the water never drains as being from teleporting to the opposite end, which graphics-wise has the water not near the hole never changing.
- Pokémon Mystery Dungeon: Rescue Team: One of the unlockable friend areas is "Enclosed Island," which houses Deoxys after recruitment. It appears as a rocky outcropping at the center of a circular waterfall out in the ocean, accessible by a single narrow path that leads offscreen.
Description: An eerie light encloses this rocky island, keeping out the sea. It is as if the island is in another dimension.
- Uru: Ages Beyond Myst: Downplayed in Ahnonay, which is an island whose large central lake is fed from the ocean by a shallow waterfall. This is one of the first clues that Ahnonay is artificial, but it disguises a strong current in the 'ocean' that prevents visitors from swimming out to find the skybox. Once you find a way to escape, it turns out the real Ahnonay is a normal example - the megastructure you've been inside is suspended from rocks jutting out of a circular waterfall that extends down and up farther than you can see. Since True Ahnonay isn't attempting to pass itself off as the lost D'ni homeworld, though, it's possible it's genuinely bottomless.
- World of Warcraft: In Battle for Azeroth, Queen Azshara exposes the undersea realm of Nazjatar to the air by using her magic to open an immense hole in the ocean bounded by towering waterfalls.
- Zack & Wiki: Quest for Barbaros' Treasure: The final location of the game, Treasure Island, is hidden in the very center of the Earth, and accessed when Barbaros punches a circular hole into the ocean and clear into the Earth's interior. The main characters get there by simply sailing their ship right into the hole.
- What If?: Subverted in an article about draining the oceans by putting a 10 meter diameter portal at the deepest point in the oceans. Putting the hole that deep wouldn't even cause a visible whirlpool at the surface.
- Adventures of the Gummi Bears: In "My Gummi Lies Over the Ocean", Gruffi and Tummi are stranded on an island in the middle of a hole in the ocean, surrounded by falling water◊. Unlike other examples, the water is filling up the hole, threatening to sink the island. Gruffi and Tummi — and a third Gummi, Gusto, who was stranded years before — have to figure out how to get back home before it does.
- Inside Job (2021): In "My Big Flat Earth Wedding", Reagan brings the Flat Earth conspiracist Harold to a Big Door in the middle of the Pacific Ocean to convince him that he has reached the edge of the world, even though in truth, the gate actually leads into the Hollow Earth. When the gate is activated, its opening creates a massive watery sinkhole that drags a bunch of hapless whales in and nearly does the same to Reagan.
- Steven Universe: The Lunar Sea Spire from "Cheeseburger Backpack" appeared as an intricately designed tower at the center of a watery sinkhole somewhere in the Northwest Atlantic ocean, glowing with an unnatural light that underscored the mystical nature of the structure. As the episode begins it is revealed that centuries of neglect have taken their toll on the structure, and the mechanism that has kept the waters from closing in on the tower is about to fail. Pearl mentions that the "moon goddess" statue that was taken from the top floor of the spire is the only thing that can reverse the damage and save the spire from destruction... but the Crystal Gems are ultimately unable to return the statue to the spire, and must beat a hasty retreat as the sea sinkhole closes up and the structure is swallowed by the waves.
- Bell-mouth spillways found in reservoirs around the world. When the water level is low, the top of the spillways look like inverted trumpet bells stick out of the water. When the water level is high, it looks like a hole has opened up in the surface of the water. One such spillway can be found at the Monticello Dam in California.
- The National 9/11 Memorial features two recessed rectangular "reflecting pool" fountains set in the footprints of the Twin Towers. At the center of each pool is a smaller square waterfall that seems to disappear underground. The edges of the larger pools themselves are much taller waterfalls that cascade inwards.