This is a paid asset, but now you can download for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
The Concave Collider is a component for Unity that enables complex and accurate collision detection and raycasts.
In Unity, mesh to mesh collisions only work if at least one of the meshes is convex.
Detail this asset from Unity Store: Original Link
Concave Collider v1.24 - Google Drive
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The Concave Collider helps overcoming this limitation by providing a way to automatically compute a set of colliders that can fit any shape; convex or not. And with just one click!
Includes 5 different sample scenes.
Requirements:
Unity Pro 3.5/4.x, Unity 5 personal or professional edition. Windows or Mac only. Products build with it will run on any platform.