Russian doll

noun2

1. •
   One of a set of hollow nesting dolls, usually wooden and decorated.

   • When she opened it he saw her eyes flash to Simon's, saw the tremulous smile and the stroke of her finger down the painted face of the Russian doll. Those dolls that had other dolls inside. — 2014
2. •
   (figurative) An issue or situation which repeatedly reveals more levels of complexity.

   • Any society is thus like a Russian doll, only much more complicated than this toy, whose inclusions are linear, i.e. unidimensional. A society can be imagined as a multidimensional Russian doll. — 2012