Portal Pool

"Digory: I think we can get out of this place into jolly well Anywhere! We don't need to jump back into the same pool we came up by. Or at least not yet." Polly: The Wood between the Worlds. It sounds rather nice."

- The Magician's Nephew

If you're a character in a fantasy setting, proceed with caution when approaching any large body of water. The most mundane-looking lake, spring, waterfall or well could secretly be a portal to a Magical Land. Your Breather at the local hot spring could turn dreadfully exciting without warning.

In some, you just keep falling until the water disappears and you hit solid ground -- always unharmed, of course. In others, you step or fall in, go down (what are those weird lights?), turn to head back up (wasn't this down a minute ago?), and find you're still in a body of water, but it's not in Kansas anymore. Such cases are not guaranteed to work both ways.

Compare Portal Picture, its oil-and-canvas counterpart. Compare also Portal Door, when doors lead to someplace non-adjacent. Not to be confused with No-Flow Portal, which can be about portals immersed in pools, nor with the pools in Portal, which will cause an unsatisfactory mark on your official testing record, followed by death. Nor this, as long as we're talking about Portal.

Anime

 * Yu-Gi-Oh! GX: The hot spring at Duel Academia turns out to be a portal to the Duel Monsters' world. Fortunately for the boys, the wayward trip comes with instant wardrobe replacement.
 * Kyou Kara Maou: Yuuri's primary conveyance between the two worlds are bodies of water. His first trip was amusingly brought about by getting his head dunked in the toilet.
 * Inuyasha has a portal well. However, it's empty, so be sure you meet the requirements for transport, 'cause if you don't make it across, it's gonna hurt.
 * Skuld in Ah! My Goddess can use bodies of water to teleport. She initially materializes while Keiichi is taking an innocent bath.
 * The manga series Shinobi Life has first Kagetora, and then others, sent back and forth between the modern era and the time period Kagetora's from, via falling into a lake. However, arriving back in our time somehow results in appearing right above and a little to the left of a large skyscraper. Good thing he's a ninja.
 * Girls Bravo
 * Fate Averruncus from Mahou Sensei Negima can use water to teleport (Evangeline and Kotaro use shadow instead).
 * 's Artifact lets her get in on the act, too.
 * In xxxHolic, purified water, such as that from an old well, can link to a pool on the mountain that the Zashiki-Warashi lives on.
 * One of the three demons lords in Demon City Shinjuku resides in an aquatic abyss, and uses the city's puddles as portal pools to pull in and drown unwary travelers. When the hero kills the demon in its own realm, all the puddles suddenly erupt as geysers, throwing him back into the real world.

Film

 * Hot Tub Time Machine.
 * In Pirates of the Caribbean: At World's End,
 * In On Stranger Tides, the pool of gravity-defying water in the cave is a portal to the Fountain of Youth, although at first it looks like the pool "eats" things that touch it.
 * Inverted in Enchanted: a fairy-tale princess is pushed down a well and ends up in the very un-magical land of modern Manhattan.
 * Constantine from the movie of the same name flat-out states that water "lubricates the transition between worlds" - meaning you can get there as long as you know how, but without water, it literally hurts like hell.
 * The 1997 movie Warriors of Virtue consisted of this when the protagonist falls into a whirlpool and is teleported to the land of Tao.
 * In Poltergeist 3, an oil slick in a parking deck turns into a portal pool, with monstrous hands reaching out of it to abduct Carol Anne, as well as her cousin and the cousin's boyfriend. The rest of her family later finds the other two chilled to the bone but otherwise unharmed.
 * The horror movie House (not that House) has several portals connected to one another through different parts of the house. The portal pool effect comes into play when Roger explores the abyss behind the bathroom mirror, and ends up falling into a black ocean that leads down into a river in his Vietnam-based Mental World. Diving into the river again causes him to surface in the swimming pool behind the house, which was the same Portal Pool that his son got lost in several years before.
 * The Water Babies.
 * The Blair Witch Project - Ten-year-old Eileen Treacle went to the first annual Wheat Harvest Picnic near Tappy East Creek. During the picnic, Eileen wandered off to wade in ankle deep water. Eleven eye-witnesses claimed to have seen a ghostly white hand reach up out of the water and pull her in under the surface. No body was ever recovered. Later, the creek mysteriously became clogged with oily bundles of sticks, rendering the water useless for thirteen days.

Literature

 * There was a Choose Your Own Adventure book that used this trope.
 * CS Lewis's The Chronicles of Narnia: The portals branching off to various worlds in The Wood Between The Worlds lie in pools scattered about the Wood. Jump into a pool, land in a world -- if you have the right magic ring, that is.
 * Interestingly, pools can also lead to "empty worlds" where nothing exists yet (allowing some characters to ), though the destruction of a world causes its pool to dry up (as happened with ).
 * Dian Curtis Regan's Princess Nevermore (much like Enchanted) features a princess from a magical land who travels to the "real" world by way of a magical pool.
 * Alice in Wonderland is a good variation of this; Alice fell down the Rabbit Hole, but there wasn't any water.
 * In The Looking Glass Wars, which is based loosely on Alice in Wonderland, the Pool of Tears is a portal to our world and certain puddles are used to return to Wonderland.
 * In Maybird and the Ever After, along with the other sequels, the titular character enters the After Life through a glowing lake hidden in the woods behind her house.
 * A complicated use of this trope is in Elisabeth Vonarburg's Reluctant Voyagers.
 * In Walter J. Williams' Implied Spaces, Pools of Life are pretty much equivalent to 'save points' in games - you can enter a pool to have a snapshot of your memories stored and/or your body plan altered and/or have yourself deconstructed then reassembled at another location.
 * To a powerful watercrafter in Codex Alera, any body of water becomes one of these.
 * Though not a still body of water, the River in Tad Williams' 'Otherland' series (particularly the second book, 'River of Blue Fire') is how the characters travel between different simulations in the Otherland virtual reality network.
 * In The Iron Grail by Robert Holdstock, Merlin descends to the otherworldly interior of a hill (the hill fort Taurovinda) by way of a well. This is a well sacred to the Celtic god Nodons, into which offerings are placed (see Real Life section below).
 * In Bones of Faerie, Liza pulls her mother, Matthew, and Allie through the lake in Faerie to get back to their village, though the lake is only a portal because Liza has the power to turn reflective surfaces into portals (into the past, present, and future).

Live Action TV

 * A Tom Sawyer-esque fantasy world (complete with rustic Earth Grandmother) at the bottom of a swimming pool appears in an episode of the The Twilight Zone.
 * Perhaps more a stylistic similarity than Playing with a Trope, but Stargate SG-1 simulates the rippling water for the Stargate. Needless to say, it does take you to other planets at least.
 * The Lost World at the bottom of the waterfall in the original Land of the Lost.
 * The eponymous Catweazle, a bumbling wizard, casts a spell in the 11th century to escape from invading Normans, jumps into a lake, and finds himself transported to the 20th century. He goes back in the last episode of series 1, but repeats the procedure in series 2.
 * Angel jumps into an empty swimming pool in Season 2 of Angel that acts a portal to another place.
 * In Charmed episode 4 season 4 Enter the Demon, a zen master can jump through any water surface (usually of a pool) to enter the plane of limbo.
 * The Buffy episode Anne has a portal to a demon dimension where kidnapped street kids are taken to be worked to death that looks like a pool of used motor oil.
 * In Fringe episode set back in eighties Peter Bishop,, tried to invoke this trope almost dying in progress.
 * Subverted in Doctor Who "The Curse of the Black Spot". The "siren" is initially thought to enter the ship through standing water so everyone hides in the driest parts of the ship.

Mythology

 * In ancient Welsh mythology, the entrance to Annwn - the Otherworld - could be located in a number of places and appeared or disappeared on a whim. One of these is a real island off of Ynys Gwales that only appears when the tide lowers in just the right way. In the Second Branch of the Mabinogion, another portal is located in a lake in Ireland. The giant Llasar Llaes Gyfnewid and his even more gigantic wife, Cymidei Cymeinfoll, emerge from here carrying the Cauldron of Rebirth.
 * In Mayan mythology, the Xibalba was a gloomy, watery underworld inspired by the cenotes that lead to flooded underground caves that dot the Yucatan peninsula. These cenotes were believed to be portals to Xibalba, and offerings were thrown into some.

Video Games

 * The entrance to the Hazy Maze Cave and Dire Dire Docks (in this case a vertical wall of water) in Super Mario 64.
 * The entrance to the Metal Cave in Hazy Maze Cave, which turns out to be behind the suspicious waterfall outside the castle.
 * While not literal, Sora's dream sequence in the beginning of Kingdom Hearts evokes the same sort of imagery as he plummets into the ocean and lands in a surreal, foreshadowing, tutorial sequence.
 * It also happens to  and   in the opening movie cutscenes for Kingdom Hearts II and Birth By Sleep respectively.
 * In Age of Mythology the Well of Urd, is one of the many ways of reaching the underworld that you encounter through the game, and the one you use in the nordic campaign.
 * The Mermaid Pools in Okami serve as the game's Warp Whistle.
 * In Neverwinter Nights, a pool in Neverwinter Wood allows access to the realm in which the forest's guardian spirit dwells. Rather than simply jumping in, however, the player must acquire a special knife and then it stab it into his/her own heart while kneeling therein.
 * Sort of present in Half Life. Glowing pools in the "border-world" Xen are capable of literally teleporting you from alien point to alien point.
 * The Lunar series employs this with Springs of Transmission which teleport the player between locations. Lunar: Eternal Blue even involves discovering that the huge star-shaped pool in front of the palace in Pentagulia is a Spring of Transmission in disguise, with the alien demi-goddess/love interest blasting the railing off the balcony and leaping in, followed by the rest of your party.
 * Not exactly the same thing, but old-school hilarious adventure game Eric the Unready featured a magical banana which could be summoned out of any body of water by speaking a magic word.
 * The Jade Passage in Final Fantasy II.
 * Clive Barker's Undying uses an enchanted pool to teleport the player between the modern-day ruins of an ancient monastery, and the medieval monastery itself. Bonus points for each side of the pool reflecting the other: in the present day, the pool's reflection shows the past, and vice versa.
 * Your first trip through one in The Game of the Ages kills you if you lack protection. You later find two further pools.

Western Animation

 * Codename: Kids Next Door, "Operation POOL": In fact, in this episode, any swimming pool combined with the proper Applied Phlebotinum can become a portal between the real world and the Mirror Universe.
 * Peter Potamus' hot tub in Harvey Birdman, Attorney at Law somehow turns into a portal to prehistory.
 * The Grim Adventures of Billy and Mandy parodies the Twilight Zone episode in "Puddle Jumpers", when Grim's scythe turns Billy's inflatable pool into a "cosmic sinkhole" that leads to several different worlds, including a bayou inhabited by a kindly old grandmother... except she wants to bake Billy into a pie.
 * The Cow and Chicken episode "The Laughing Puddle": people are disappearing one by one into a creepy laughing puddle, until Chicken is the only one remaining. With great trepidation he jumps inside -- and
 * Courage the Cowardly Dog featured a sea witch of sorts who used these to transport back and forth in between her magical lair.

Web Original

 * SCP Foundation has SCP-354, a lake filled with a red fluid. If you lock yourself into a submarine and keep going down, and down, and down, the fluid will keep getting thicker and thicker... until you finally reach a whole new world.
 * They also have SCP-120, a pool that cycles through 11 different destinations. To choose a particular destination, Class D personnel have to be used. Why? Five of the eleven destinations are the Lagrange points. Which are in space. And the pool only works with living, awake humans.

Web Comics

 * Alice from Cheshire Crossing uses mirrors as portals to Wonderland, but once used a puddle of water as a mirror.

Real Life

 * Celtic and other British isles tribes believed that lakes and other bodies of water were portals to the spirit world. Archaelogists occasionally encounter finds of worked silver cauldrons, containing many valuable artifacts, that were deliberately dropped in lakes or bogs as offerings. Both cauldrons and lakes also pop up frequently in the preserved Celtic and Welsh legends that have survived to today as portals to the underworld.

Other

 * Subverted in a Choose Your Own Adventure article on Magicthegathering.com. In it you journey through the horrible world of Shadowmoor, a Bizarro World of the idilic world Lorwyn. At one point you encounter a pond in a cave and the description suggests it might be a portal pool to Lorwyn. If you enter it however, you are promptly ripped apart by vicious Merrows, and the game calls you a naive idiot.