Another way is to use the “horizontal” hole by using the Landscape Visibility Mask node in your landscape material. Here is the documentation
When setting up the LandscapeVisibilityMask node make sure this is in your landscapes material and not a separate material. The images there do not indicate these are the same material. Once you have the material setup in the details panel for your landscape on the right hand side you’ll want to add the landscape material you made again in the “Landscape Hole Material” slot. This will allow you to use the Visibility sculpt tool to paint areas that are holes.
If you have any questions let me know!
Tim