right over the top of the cave, yet underground place a cube, just for shadow casting, stretch it so that it’s shadow will cover the whole cave. If the cave shape is too hard to cover you can use a few cubes or model a custom object to do the same. The point is that the terrain and caves are not properly casting shadows b/c it is all one object. Make other objects to help the engine know where to put things in shadow. Does that make sense? Just grab the generic cube from the unreal assets or make your own the point is a cube is easy for a light to know how to cast a shadow from and your caves will then be in shadow like you wanted them to be.