Terrain Tool Errors

The smooth tool seems to create another hollow layer under the surface and then the top surface becomes uneditable.

I tried to fix this with the add tool to fill it in and something happened to where now whenever I look towards the ground in a certain spot my screen completely blacks out.

Finally managed to get the black screen spot to go away by adding and removing land until the glitch disappeared.

It seems to be the smooth tool creates the hollow bit underground when there's grass and it doesn't know how to smooth it out...