Osage Orange

I found this strange fruit while running down Bebek Dağ. A noble street dog was defending me from the barks of an owned dog when one of these strange fruits fell from the tree. At first I thought it was a tennis ball, but then I found a pile of these.
I wish I had taken a photo if this street dog, but I didn't. When I looked back up the hill, he was sitting, looking at me and gave me a small wag. I ran home with this grapefruit sized fruit for further investigation. Apparently, it doesn't taste that good.