The truly terrifying part is when the world have almost entirely turned into Instability blocks(the red, blue, green spreading blocks) and starts falling apart like gravel... Omg where did I put that Linking Book!
There's also black and white decay. Last I checked, the red, blue, and green ones would only spread through solid blocks (i.e. everything save air) and do NOT fall, white spreads through air as well AND damages you when you walk on it, and black is like the colored ones, except it periodically
eats the block below it, then falls like gravel and drags all the blocks above it down to fill the space it just created and any open space below, so you eventually end up with lots of giant holes into the Void, which continue to grow until there's nothing left save maybe some floating islands, tendrils, spheres, roofs from decayed buildings, and tree branches.
Back in the days of MC 1.2.5 (FTB Retro!), black decay was the only type of decay that existed. If you made an age with too many Dense Ores symbols (which people did pretty often, because Equivalent Exchange 2 and ALL THAT EMC), you would start to see craters appearing on the surface all over the age within minutes, and if you stayed in one place, there would be nothing left in a matter of hours.
However, black decay is much more rare now that the other colors of decay, potion effects, meteors, random explosions, the "sun scorned" thing, enemy regen, and who knows what else have been added.
