Casting defect - any idea what happened here?

That is what it looks like to me. I see them often from foreign materials in our sand. Wood chips would be a huge source of gas if it ignited and would cause this defect while the casting was still mushy or not solidified completely. Looks too uniform to be just a cold shut type defect. Another crazy possibility is sweat...just a drop in that spot...I have seen issues with sweat drops causing gas bubbles in hand molding operations. It usually is accompanied with some type of gas discoloration around it but not always.

 

I'll admit I never considered sweat , just because I know not to let it happen . wear a hat, blow out those molds if you drip in them

V/r HT1