I've gone to the end (Zone 5 on 8th area of Death Marsh, a.k.a. "Pad 9") and beyond "Ahead 5" which is what you are looking for, I have unlocked "Halfway 5", "Rearmost 5" and "Lance 5" also. I'm sorry that I don't remember how often each formation became available. But there's a good chance that you will be coming across it soon.