What is "shell ice" and how do I get rid of it?  Good question, here's what I know...

Shell ice is a condition that happens when the surface freezes leaving an air pocket between the previous surface and the new one.  It breaks up rather easily and leaves craters in the ice if not dealt with.  It can be caused by several things, one of which is excessive wind during a resurfacing.  Another would be when the temps are not cold enough to freeze the layer of water evenly.  Yet another would be from the temps being so cold that the new layer doesn't adhere to the previous causing a shell on top of the previous surface.

 Now the trick is, what to do with it?

1. You can break it up, clean off the shrapnel, put down a flood or two of hot water and watch it melt into the surface and disappear.
2. You can also poke tiny holes in the shell, flood with hot water and wait for nature to do it's thing.  The holes allow the water to fill the gap in the shell and then it "should" freeze solid again.
3. You could break it up, clean up the shrapnel, fill the craters with warm water, wait for that to freeze and resurface over the entire rink to get the surface smooth again.

Any of these will do the trick and I am sure there are other solutions as well.