I got it!!
It was in a "painstakingly, well prepared, watertight crate"
I wrote in my file that in this crate were some infants, and I think it's true but it can be also some diapers! And I think it happens when the population has reached its maximum (notice on the picture, I'm at 90) With some lil kids in the crate the population would reach 92 or 93 peoples!!