To Those who don't know. Nuketown map in Black Ops4 has a secret thing - zombie mannequins.
What you need to do is - in short period of time you need to shoot mannequins heads off, from all of them. Then you will hear the warning sound and voice starting to count down. After it counts down to 0, then the hatch will open in the middle of map (where are train waggons) and you will see the rocket lauches up in the sky and blows. And here start all the fun, mannequins will start falling from the sky and tries to kill you.
I and oskaras91 did it in offline play mode.
Nuketown map you can find under Bonus tab.
You can watch us doing it in our video on youtube here -->https://www.youtube.com/watch?v=Yk_y7sAZ-yc
If you have any questions, please ask :)