Science Destroys Magic

So, we have a world. The world has magic in it. However, this world isn't a normal fantasy setting and is instead more or less the real world. Well, then what happens to magic or magical creatures? It turns out that they disappear, for better or worse.

It seems that magic and science simply don't go well together, so that as one grows stronger, the other fades. Thus, the further humanity progresses in either science or civilization, the weaker magic becomes. In some cases, magic may even die outright.

This may be a factor in The Magic Goes Away. Compare Death of the Old Gods. Compare and contrast Magic Versus Science and The Magic Versus Technology War, which involve more direct conflict, possibly via force.

Anime

 * Side material for Mahou Sensei Negima reveals that magic may be doomed. Magic is largely dependent upon a theory of the way the world works based on such things as the classic elements or fantasy creatures. However, as science progresses, it becomes more difficult to hold the proper mindset. It is implied that eventually magic will have been replaced completely by science.
 * In To Aru Majutsu no Index, the power of magic and the various churches built around them have largely declined, leading to a balance of power that is roughly even between that of the Science and Magic sides. While science continues to grow, magic largely stays the same. Quite interesting when one considers magic came into existence as an attempt to replicate rare but naturally-occuring Psychic Powers in ordinary people, an alternative everyone could use. The balance started unraveling when science discovered how to create espers through artificial means (the Power Development Program shown in the series).
 * The second season proves that this is more serious than what one might suspect. It was already known that espers can't use magic or they'll die but it turns out the opposite is also true: a sufficiently powerful AIM dispersion field such as the one wielded by will slowly kill anyone who attempts to cast spells within the field's range aside from No Selling any and all magic directed against the wielder.

Comic Books

 * In DC Comics, Doctor Thirteen is a paranormal investigator who goes around debunking magical phenomena as mundane activity. Since he's in the same verse as The Spectre and The Phantom Stranger, it didn't really make sense and was retconned into being that Doctor Thirteen's very presence makes magic stop working.

Fan Fiction

 * In the Harry Potter and The Methods of Rationality setting, it is common knowledge that The Magic Goes Away: No-one can surpass the Founders of Gryffindor, who can't surpass Merlin, no one can make stuff as good as the ancient artifacts, etc. According to the Blood Purists, of whom the Death Eaters are a violently proactive faction, it's the fault of the muggle-borns for watering-down wizarding blood. Rationalist!Harry gets Draco to collaborate with him in investigating the true cause of the decline of magic through the ages. One of the alternate hypotheses considered is this very trope: "magic disappears because of Muggle science developing".

Literature
""Yes," said the ghastly passenger. "You are English and the English believe!"
 * Lord of the Rings - elves, ents, dwarves, orcs etc are fading away as "The Third Age of the world is ended, and the new age is begun"
 * In Stardust, all the fantasy elements of our world largely left for another world. There are points of contact between the two worlds, but even those are both becoming rarer and the times when people cross between become rarer as well.
 * A lot of Victorian literature is constructed around this idea. Dracula stands out as a great example as the mysticism of Eastern Europe is trumped by the science and innovation of an industrialized England.
 * In Conan the Barbarian, according to the good sorcerer Pelias Conan's kingdom brought forth an age of logic and science which is slowly destroying the magic.
 * In Voyage of the Basset (televised as Voyage of the Unicorn), the motto of the Basset is "Credendo vides", translated in-universe as "By believing, one sees." In other words, you have to ignore science and believe in the fantastic without evidence before you can see behind The Masquerade.
 * In Magic Kingdom of Landover, not believing in magic and believing in science kills off magic in The Verse, the "real" world has nearly no magic as a result.
 * This is the end goal of the maesters in A Song of Ice and Fire. There's a conspiracy that wants to destroy magic in favor of science.
 * The War of the Flowers invokes this. The Evil Overlord of the Faeries says that this is why life sucks in their world, but he may be lying.
 * Seems to work both ways in the Kate Daniels books: Technology destroys magic and magic causes technology to fail.
 * Inverted in The Dresden Files. There active magic destroys technology to the point where cell phones are considered the canaries of magical activity.
 * Ray Bradbury's short story On The Orient, North featured a ghost fleeing continental Europe to save himself from the scientific intellectuals that were "killing" him, aided by an English nurse.

"True. Better than Americans, who doubt. French? Cynics! English is best. There is hardly an old London house that does not have its sad lady of mists crying before dawn.""

New Media

 * In Suburban Knights, the premise is about

Tabletop Games

 * Mage: The Ascension. This is what happened to Earth in the campaign setting. The Technocracy managed to impose its scientific, rational world view and thereby suppress the fantasy elements (e.g., magic and magical monsters) that were common before.
 * Changeling: The Dreaming tends to assume science is Fae-smothering Banality, though the Nockers might disagree (indeed, the largest infusion of Glamour in recent history was the moon landing).
 * In Magic: The Gathering, this was basically what Yawgmoth hoped. Phyrexian 'technology' seems to be Magitek, Mechanical Lifeforms or a combination of both, however, so this may not be a full example.

Video Games

 * One late conversation in Arcanum mentions that the world goes through cycles of magic, uneasy balance, tech ect, with the game taking place during the uneasy balance leading into an era of technology.
 * In a more literal sense, advanced technology weakens magic (admittedly, the opposite is also the case, with high magic disrupting technology).
 * Touhou. The main setting, Gensoukyou, has actually been set up to absorb things that would be destroyed from lack of belief.
 * The Extended Universe for King's Quest has this as part of the games' backstory. The magical creatures and beings that once lived on our world were Genre Savvy about the matter, so when we started becoming more scientific they voluntarily left to a new world before any weakening could occur.

Visual Novels

 * The explanation the fantasy side gives in Umineko no Naku Koro ni for why they don't use magic openly is that humans have become resistant to it as they acquired an 'anti-magic toxin'. Thus, in large groups it becomes impossible to use magic on them. The mystery side argues, of course, that magic doesn't exist in the first place. It is largely ambiguous as to who is correct.
 * In Shikkoku no Sharnoth, as the light of civilization grew, the dark kingdom of the fantasy realm willingly left to make room for them, as they could not exist in it. The only one who stayed was the land's powerful king, who stayed inside his dark castle alone.
 * In Fate/stay night, there is an expression: As technology moves towards the future, magecraft moves towards the past. As technology has advanced, the ability of magic users has only declined. Tsukihime, set in the same continuity, originally introduced this idea with the fact that where magic was once powerful, it now has to hide from society at large or risk destruction.

Web Original

 * The Administrator's note for the SCP Foundation has this as explaining why we don't have all these weird cults dedicated to SCP-level objects. Not that it seems to have worked...

Western Animation

 * In Flight of Dragons, the central premise is a sort of Elemental Rock-Paper-Scissors in which magical creatures can't exist in a world where science dominates, and civilizations based on science are destroyed by fear and superstition.