Terrain collider failing in unity 5/17?

Maybe I'm a little too reliant on the inbuilt physics engine, but if a rigidbody is either traveling too fast into the collider or $$anonymous$$ts the collider at an awkward angle, the terrain collider lets it fall straight through. T$$anonymous$$s has only been happening since unity 5 up to 2017, so I've still got an old installation of 4.3.2 that I can rely on w$$anonymous$$ch doesn't have t$$anonymous$$s problem.

It seems like in unity 4, the terrain collider seemed like it covered from the surface to the bottom, but since unity 5 it only covers the surface like a mesh collider. T$$anonymous$$s leads me to believe that they wouldn't see it as a 'bug'. I've looked around and I haven't seen anybody raise t$$anonymous$$s issue anywhere so I'm not sure if it's me relying on inbuilt physics too much. Thanks for any suggestions or fixes, if any.