Reminds me of a story I read a few years back, the players come across a portal about twenty feet off the ground that had salt pouring out of it. They were supposed to leave it alone but a player realized they could sell the salt for mad stacks. The next 6-7 sessions was them setting up a "factory" with workers and mercs to fight the monsters that also came out of the portal, which the DM made because he wasn't really thinking this would have happened as was trying to nudge them back to the campaign. I don't remember much else but I think the DM eventually had the portal collapse and they flooded the market with so much salt they ruined the value of it.