Hi,
My question is can you somehow create caves in the landscape? It seems that the sculpt tool only allows to make hollows - polygons go either up or straight down.
How do you make the underground level without having to put an entrance to it with a loading process?
Just create a hole -> with the landscape tool and then:
-either you build your cave with rock meshes -> take a look at the effect demo
-or you create a cave mesh + after that add the details in the editor 
Imagine a dune in the desert and a level inside this dune that you can freely enter and exit.
Yep, it still works the same as mentioned above
(my incredible good drawing skills :p)
black -> landscape (dune)
blue -> rocks to hide the seams at the entrance
red -> cave mesh
or do everything with rocks
Ok, but how do you create a hole in the landscape, all I found was delete tool which deletes entire sections, how to erase polygons from it?
Take a look at this page: https://docs.unrealengine/latest/INT/Engine/Landscape/Editing/SculptMode/index.html#visibility
Yeah, just discovered that stuff. Thanks