Rigidbody and terrain problem

Here is a simple cube (with a box collider and a rigidbody) and it falls through the terrain...the cube is placed in mid-air and I've no idea as to why this happens.

The terrain has a simple 'terrain collider' attached to it and nothing else - no colliders/scripts etc.

The terrain has length and width set to value 80000 - in case that's of any use..

Here is a screenshot with the cube selected...