With Earth of the first half of the 20th century being a possible setting for future adventure paths and modules, I would like to start this thread for talk about some events which esoteric and unusual nature make them perfect hooks for scenarios dealing with adventurers of Golarion getting too involved into the history of the strange humans only world.
The Angels of Mons:
http://en.wikipedia.org/wiki/Angels_of_Mons
In the alternate Earth of Golarion Prime Material plane, daemons serving the Horseman of War are waiting for a battle which would unleash enough psychic energies for open a durable portal to Abbadon by nihilistic cultist who are serving as solders in one of the armies which are about to clash. The PCs are send to Earth to stop that concrete bloodbath somehow by command of an Empyreal lord.
Cottingley Fairies:
http://en.wikipedia.org/wiki/Cottingley_Fairies
In the real world we know the photographs where fake but what if in Pathfinder's Earth the two young girls just witnessed an incursion from the First World? The PCs could find themselves being drag to Earth by a capricious powerful fey who demands them bring a group of rebellious fey back to the First world and then close the rift leave in Earth, teaming up with a very special NPC during their mission, Sir Arthur Conan Doyle.
The encephalitis lethargica epidemic:
http://en.wikipedia.org/wiki/Encephalitis_lethargica
From 1917 to 1928 a strange disease nicknamed "sleep sickness" affected great part of the world, slightly eclipsed by the 1918 flu pandemic. This scenario would deal with an alliance between the Denizens of Leng and the daemons serving the Horsemen of Plague in their attempt to wake up an Old One being sleeping in the deeps of the blue planet. The PCs would start their journey in Ustalav, in Carrion Hill, being forced to make deals with cultist of Yog-Sothoth so they can travel to nightmare plane of Leng and then end their journey on Earth, facing the horrors of a rising City of R'lyeh.