Ue4 landscape is the Homer Simpsons spice rack solution.
You can build on it, but you have to be aware thats the base you start from.
You can try to make the hole not a square with the retopologize tool.
Make the hole much larger than you think.
Export the landscape to a mesh.
Load the mesh in a DCC.
Create the entrance that merges with the terrain however you like.
- You can either keep the coordinates in place to where the terrain loaded the hole or move the final result to 0,0,0 the difference is you have to move the object in engine or just drop it into the level.
You can achieve OK looking things in spite of the spice rack you start with.
But to guarantee characters / things not getting stuck or procedural items not spawning you really need a much larger hole than what you think - which means a mesh has to take over of some portion of the landscape.