How to smooth a mesh? I have this terrain mesh with some mountains, but also some plains between the mountains.



I would like to smooth out the plains, because when 3D printed they cause a lot of unnecessary walls:



Basically if you'd represent this as an image with white = high and black = low, I'd like to blur the image.

I tried the "Smooth Laplacian" modifier which does nothing and the "Smooth" modifier which does this:
Apparently there was something wrong with my mesh (although it looked completely fine in edit mode). I activated the Mesh: 3D Print Toolbox add-on and then used the Make Manifold button on my mesh. After that all the smoothing tools (Smooth modifier, Smooth sculpt brush) began to work properly.