So I got this morning thinking about the Deccan Flats and Siberian Traps. These are both areas that witnessed massive volcanic eruptions some 250 mya and 70 mya. The eruptions in each area lasted hundreds of thousands of years. (well as best we can tell). These events are really cool and may have been contributed to the mass extinctions that occurred during those times as well.
So one could drop these types of events into your world and create a weird and really cool adventuring area. Think of demons and devils and other creatures trying to make their way into you world.
And volcanoes make really cool caves.