The recent Call of Duty's have all had some sort of major nuke-like killstreak. Black Ops 2 was very different because at first there seemed to be no nuke killstreak. There were no options for it in the scorestreaks menu. However there is a nuke killstreak which is at first hidden.
The only way to unlock the nuke scorestreak is to get a 25 killstreak without any other scorestreaks on. So that's 25 consecutive kills with just your gun. Some say you need to do it on the Free-For-All game mode specifically. But I think it may work in other game modes too.
The first person to unlock the scorestreak was actually a Machinima director. Here is the video for proof.
But this does raise another question. There is still another slot between the Swarm and Nuke scorestreak which no one has yet discovered or known how to unlock. Hopefully this slot will also be discovered soon.