We know the common tropes for an apocalyptic event: meteor/asteroid hits earth; crazy infectious virus/bacteria; AI takes over; we nuke ourselves; climate change; hostile aliens. But what are some other, less known but still plausible, ways an apocalyptic event could go down.
- Human populations must experience a devastating blow (say at least 75% dead, and does not need to be evenly spread out) or be completely eliminated
- The blow to humans can be direct (e.g. radiation poisoning) or indirect (e.g. starvation because no arable land)
- All other plants and animals may or may not be affected (this is unimportant)
- This has to be somewhat plausible according to today's science (e.g. no time-traveling humans from the future, no we all get sucked into a parallel universe)
- This can't be a super popular or well known apocalyptic premise (e.g. no hostile AI takeover)
I'm looking for things like: Phytophthora, a water-mold genus that affects a wide range of food crops (think potato famine) mutates and destroys crops worldwide. Or a rogue planet enters the solar system and takes earth out of orbit. Or the whole gamma-ray burst thing. Or a super-volcano goes off. Basically anything that is somewhat scientifically plausible but not (very) common knowledge. Please also give evidence as to why this event could be feasible (though I would also be interested in well theorized events without as much evidence)
EDIT: to prevent this from being viewed as a subjective question, here are my objectives for accepting an answer: I will accept the answer with the most plausible theory (ideally with references or past examples), while still being relatively unknown (see the first paragraph of my question for 'commonly-known' apocalyptic events). I understand many apocalyptic possibilities have an unknown likelihood (e.g. the Fermi paradox illustrates how we have no clue how likely aliens are to exist) but some (e.g climate change) are currently more likely than others (e.g. hostile aliens - given we don't have any indication of aliens visiting earth in the millions of years its been here) and we can predict the likelihood of many events (e.g. how often super-volcanoes go off, how many are on earth, when they last erupted etc.). I also don't want answers that have lots of steps that each have their own likelihoods (e.g. the plot of the movie Life: life found on Mars (some degree of unlikely), it grows and becomes/is sentient (unlikely), it becomes harmful/malevolent (unlikely) it escapes to earth (again another step) where it can rapidly spread throughout the ocean (another step) and kill everyone.)
EDIT 2: PS even if it makes the mods grumpy I do like it when people come up with crazy/'bit of a stretch' answers (who knows maybe they will help someone who is reading this with their own world-building), I will upvote them if they are still (somewhat) scientifically plausible and not commonly known.