There are some places where the barrier between worlds is thin, allowing people to easily slip from one universe to the next (not always on purpose) and occasionally causing parts of one world to physically manifest in the other. Often these places are a permanent, fixed geographic location (The Bermuda Triangle is a frequent target), but occasionally they will exist only temporarily or even move around. Occasionally involves (comparatively) minor anomalies in the laws of physics (such as compasses not working correctly) and/or copious amounts of fog. Expect it to be described with some near-verbatim variation on the phrase "where the barrier between worlds is thin".
Though this is mostly a science fiction trope, it tends to crop up in fantasy whenever The Fair Folk are involved.
These often connect to a Alternate Universe, Inn Between the Worlds, or the Void Between the Worlds. May also be a Place of Power because of supernatural forces being more concentrated there. Compare and contrast Portal Door and Portal Pool, when the threshold between worlds is much more discrete. If the dimensional barrier is completely breached in a area, then see When Dimensions Collide.
- In The Sandman, "soft places" are spots where reality is weak leading to easier inter-dimensional travel, and time working oddly. There is one in the Desert of Lop in China (a real place).
- Superboy and the Ravers: Shaar Q has the ability to sense places where jumps have recently been made into sub-dimensions and reach through or traverse into them.
- A Chance Meeting Of Two Moons: The Door to the Realms In Between, which draconequui can open to allow them to visit alternate realms, including the dream world. A Chance Meeting of Two Moons marks its first appearance in Evilhumour's works; it's later mentioned in The Mare From the Moon as a sign that more worlds than these are connected.
- Ralph Bakshi's Medium Blending feature Cool World has Doctor Whiskers create the Spike, which tears a hole in reality that allows Doctor Whiskers to venture into the "noid" world (ours) and draws Frank Harris into the cartoony Cool World (theirs). The Villain is The Vamp Holli Would, who wants to venture into the "noid" world herself, in order to experience a real orgasm, among other things. She succeeds in capturing the Spike, which causes the Toon Physics of Cool World to flood into the Noid World and vice versa.
- In Terry Pratchett and Stephen Baxter's multiverse-centric The Long Earth series, places like this allow people with natural Stepping abilities to cross from one earth to one several Steps down the line without crossing through the intervening earth, which is ordinarily impossible. This only works for people with the natural ability to move between worlds without a Stepper box. Everyone else must take them one at a time.
- In Ciaphas Cain: The Emperor's Finest, the Reclaimers Space Marines are able to track the space hulk Spawn of Damnation between star systems by locating the weak points it leaves in realspace when it transitions into the Warp.
- In Discworld, there are "thin places" where other realities brush up against the Disc. One is contained by The Dancers, a Circle of Standing Stones saturated with meteoric iron, and causes no end of trouble in Lords and Ladies when The Fair Folk start escaping it.
- In The Wheel of Time's Age of Legends, one such site turned into the volcanic Eldritch Location Shayol Ghul after scientists accidentally touched the power of the Dark One, who was trapped outside reality. Millennia later, it's misremembered as the physical site of the Dark One's prison. When places like these start temporarily manifesting, it's a sign that the Dark One is almost ready to break free completely.
- His Dark Materials describes the Aurora Borealis as a place where the borders separating the universes is the weakest, enabling the heroine to see a city in another world and for her father to build a bridge to it.
- "Thinnies" show up throughout the works of Stephen King. In his The Dark Tower Saga, they're explained to be a symptom of the ongoing collapse of the multiverse.
- The barrier between the real world and the Tenebrae in Knights of the Borrowed Dark is always fairly thin, but in some places they overlap; at Os Reges Point, the worlds almost coexist, and Seraphim Row is in constant danger of falling into the Tenebrae.
- In the children's novel Finders Keepers by Emily Rodda, there are spots all over the place where the dimensional barrier is thin, and this is the explanation for the phenomenon where you lose an object and then find it again in a place where you could have sworn you'd already looked: the object fell through a gap into another world for a little while before reappearing back through the gap.
- In The Secrets of Drearcliff Grange School, one of the secrets is that the school is built on a place where the barrier between dimensions is thin, which becomes important during the climax.
- In Ruin Of Angels, the city of Alikand was destroyed in a battle between gods and sorcerers, which caused significant damage to the fabric of reality. A new city, Agdel Lex, was built on top of the ruins. Trouble is, the combination of reality damage and colonialism (the surviving Alikanders weren't too happy about the empire that built Agdel Lex claiming their territory) has resulted in the existence of two separate cities occupying the same space on two closely entangled layers of reality. There's also a third layer where the initial battle rages on eternally, frozen in time.
- Lexx has Nexus Points, places where it's possible to cross between the Light Zone and Dark Zone, the existing universes.
- In the Star Trek: The Original Series episode "The Tholian Web", the crew of the Enterprise accidentally stumbles upon one of these in space while investigating a recently dead Federation ship. Kirk gets trapped in the other universe, while distortions in the laws of physics start to prevent the Enterprise's systems (and the crew's brains) from working properly.
- In the 21st-century Whoniverse, there are various places where the Doctor popping in and out too much has led to weak spots, which are then used as an excuse for spin-offs stuck in one place. In particular, the Cardiff Rift in Torchwood, and the rift at Coal Hill School in Class (2016).
- In Stranger Things, when Eleven opens a portal into the Upside Down, several of these are created elsewhere, such as in the Byers' house.
- Warhammer 40,000:
- The minds of untrained psykers are the main reason they're hunted down and captured for indoctrination as soon as they can, they can all too easily become portals for daemonic invasion (and even with training it's a possibility), or worse, learn to master their powers and become a servant of Chaos.
- The closer to the Eye of Terror, the thinner the barrier between reality and the Warp becomes, until they finally intermingle, causing Reality Is Out to Lunch.
- In the New World of Darkness, Verges are sites where the barrier between the physical world and Shadow Realm has been worn away completely. Some only open when a particular condition is met; others allow anyone to wander between worlds, usually with very bad results.
- Dawn of War: In Retribution, the Chaos faction uses Warp rifts as their method of getting from planet to planet (the others use teleporters or upload their consciousness to their Hive Mind)
- In Dragon Age, the Veil between the physical world and the Fade is weaker in some places than in others. This is especially true for places where a large loss of life or suffering has occurred, such as Blackmarsh in Dragon Age: Origins – Awakening or the entirety of Kirkwall in Dragon Age II. The most obvious consequence of the Veil weakening is that it allows demons to enter the waking world much more easily, but it also enables much easier entrance to the Fade, as well, even for people who are normally incapable of doing so (e.g. dwarves, who normally cannot enter the Fade at all, have been able to briefly do so in both Blackmarsh and Kirkwall).
- In Pokémon Sun and Moon, The Alola region is host to a rare phenomenon known as the Ultra Wormhole. On occasion, creatures known as Ultra Beasts have emerged, requiring the intervention of the island's guardian Legendaries to repel them.
- In the Touhou series, the Road of Reconsideration and Muenzuka are places where the Great Hakurei Barrier between Gensokyo and the Outside World is particularly weak, and youkai take advantage of this in order to prey on the hapless Outside World humans who get brought to Gensokyo by barrier fluctuations.
- Pac-Man World: In the third game, a new Big Bad named Erwin is creating energy siphons to drain energy from the Spectral Realm. In theory, these siphons can be set up anywhere, but one particular area where he'd set up a siphon is later explained by Mission Control as making a kind of sense, as the barrier between dimensions was thin there. This doesn't seem to have any effect other than to make the siphon's operation easier for Erwin, though.
- In Stellaris the Jump Drives of younger races tend to create such places. If the barrier actually rips, the Unbidden can enter and turn liveforms into energy.
- Neverwinter Nights 2: Mask of the Betrayer takes place partially in the Plane of Shadow, which is reachable by portals that appear at night in areas of the world where the planar boundaries are weak.