I'm currently working on the pathfinding section for a game project. I have a problem with baking my navmesh - it didn't cover all of the area I wanted to make walkable.
Here is an image of the problem:
How can I cover the entire area with a navmesh so it is walkable by agents?
Unfortunately, Unity's navmesh-based navigation system is only designed to work with terrain aligned to the y-axis - that is, the world's y-axis is used as the up vector, and the slope of the terrain is calculated using that as the reference.
The reason why only the top of your spherical object is considered walkable is because the slope of the rest of that terrain, if you consider the y-axis as the up vector, is greater than allowable by the navmesh baking settings.
The only way to solve this is by writing a custom solution - but hey, you're fortunately not the first one to pose such a question. The generally accepted approach is to use the A* algorithm with a custom heuristic function (based on the vertices of your mesh) to calculate the shortest path to a destination.
Here's a good tutorial on how to implement A* in Unity, which will give you an understanding of the basics so you can work out how to apply the algorithm to a terrain that wraps around on itself. If you're not planning on writing your own solution, there's also an existing A* asset for sale on the Unity Asset Store (free version also available), which is considered a solid improvement over Unity's own navigation system, and can be applicable to a non-vertical terrain such as yours with some adjustment.
Hope this helps! Let me know if you have any questions.
