Maybe they will unleash a hord of giant beasts they can't stop. That certainly would be traditional. Unfortunately the other choice is they will find and loot a cache of the precious materials the MC's crew would have needed. But the worst outcome is a combination of both: they will loot the place but also release the monsters. Then they will just flee with the riches, leaving the task of dealing with the monsters to the MC, but for no reward since the place would have been cleaned out already.