I built a layer of cobblestone, covered it with grass and gravel, and then lined the underside with Glowstone panels (FMP). Now some of the cobble and dirt has disappeared. It's SSP so no-one else has messed with it. Those panels were definitely laid on the underside of the cobblestone.
![2013-08-21_23.00.43.png](/proxy.php?image=https%3A%2F%2Fdl.dropboxusercontent.com%2Fu%2F8129635%2F2013-08-21_23.00.43.png&hash=e922f37be6989520fcfe0514a44152a6)