known bug with a possible workaround.
go to the atmosphere area, click on the atmosphere node. on the quality tab, click enable ray traced shadows.
the render will take a bit longer to do, but it wont show through the ground.
basically in some of the shortcuts they take in the normal rendering it doesn't quite always render the sunlight correctly through land. Forcing it to raytrace (and thus taking more precise and accurate calculations) fixes it.