As an example, in the Earthdawn setting, civilization was segregated because of demonic creatures known as the horrors that scourged and ruled over the surface world. The tides of magic that enabled them to invade Barsaive receded, and so the threat that kept people underground was gone, and a new age of adventure arose.
What sort of events drives folk towards the life of adventure? The sort of setting I'm thinking about is that in which the player characters are not the only adventurers, let alone heroes in the world. Are there any published examples worth checking out?
Update: I think I need to narrow down the criteria. I'm trying to figure out an event to launch a campaign that drives not just a handful of adventurers, but entices thousands of mere mortals, many of whom are likely to live thousands of miles away from one another to risk injury and possible death for rewards. Earthdawn falls perfectly into that criteria. I've been busting my mind while trying to come up with another scenario. So far not much is coming to me. I really liked Rob Lang's idea though! Discovery of an unknown civilization/land sounds like it may be just the thing the more I think about it