The end of the world, in any form. Global Warming/Meteorite colliding with the Earth/Apocalypse/Alien Invasion/Armageddon.
It managed to creep in to my head more or less every night before I go to sleep or if I'm sat thinking about nothing in particular.
I refuse to watch films like The Day After Tomorrow, 2010, Cloverfield, Independance Day, War of the Worlds etcetc..
It's 'plagued' me ever since I found out that the world
could end, well human life on Earth could at least. I don't think I'll ever get over that worry :| haha.