Objects appearing in Game and Scene view, but not when pressing play to preview...

Hi there, I'm a Unity newbie so I have a problem that probably is blindingly obvious to solve, but I can't see how. I have created rocks and imported them. They exist in scene view, game view and hierarchy, but when I press play and move around with my camera, they don't exist in the game!

I have trawled google and the forums here. Everything is on the default layer (as far as I can tell), I don't think anything is being culled.

Any help will be greatly appreciated! :)