Well.
Since you’re probably never gonna see them again.
I’ll tell you. 😛
Well, they are pretty safe. The only way to get to the basement is to use an elevator. The higher level zombies will not take too kindly to them as they will be more intelligent. The Tank could probably enter the elevator and use its weight to send it flying down into the basement.
The zombies aren’t the problem however; its their food supply. They can’t stay living like that all year. So, should they not bother, hunger or thirst will be their killer.