Default ground plane?

Is there some sort of default ground plane in the physics engine?
My skeletal mesh collides with an invisible ground plane at Z = 0.
My actual ground is below this at -95 but the simulated skeletal mesh drops and stops at 0.
Cloth objects on this character also seem to hit this invisible ground plane.

Nothing default.
the only limitation is a killz value.
once you reach said value the engine deletes the item. Or tries to.