The Big One
This is the way the world ends
Not with a bang but a whimper.
Or not. Bang scenarios do seem pretty likely too.
From time to time, catastrophic scenarios have occurred that put major dents in our planet's living population. Fortunately enough, none of them seem to have utterly depopulated the planet, at least in the last 3 plus billion years.
There are cosmic scenarios, like a well aimed blast of particles from a suddenly active nearby galaxy nucleus, or a close supernova, solar system variants, like a collision with a Sedna scale planetoid, or purely terrestrial events like super volcanos. The Yellowstone Caldera is a good example of the last.
Supervolcanos are fueled by gigantic magma hotspots that bubble up under the Earth's crust. One such hot spot fuels the Hawaiian Islands, punching up one volcanic island after another as the oceanic crust slides over it. The Yellowstone caldera is a continental variant. Continental variants seem to be more menacing, possibly because the much thicker continental crust allows a lot more energy to accumulates under it. It seems that good old Yellowstone has about 2.5 times as much magma under it as previously thought. When it erupts, it will be disaster on at least a continental and probably global scale. Much of the US would be buried under tens or hundreds of feet of hot ash.
If the Yellowstone supervolcano were to blow today, the consequences would be catastrophic.
Not to worry though, it only erupts every 600,000 to 800,000 years. And it has only been 640,000 years since the last biggy.
UPDATE: Still a piker at 600 km^3 vs. Lake Toba at 2,800 km^3 or so. And it only wiped out most of the human race. Though the last biggie from Yellowstone did apparently blow big chunks (of rock) all the way to Europe.