The simple answer, as I understand it, is the the human visual system is conditioned to expect light coming from above, including from the top of a picture. If light comes from the bottom of a picture, it casts a shadow on the lower (in the picture) half of the crater, where it would be cast on a bump if light was from the top. With practice, you can mentally invert the "bumps" into craters. Or just rotate the picture 180 degrees!
ETA: Here's another old thread, in which I give the same answer! And one from the BA himself.
Last edited by Trebuchet; 2014-May-05 at 06:04 PM.
Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.