We solve a problem of Tarski, and his co-authors Andréka, Henkin, Monk, and Németi and confirm a conjecture of Németi. We show that for any pair of ordinals 1 < α < β, the class of α-dimensional neat reducts of β-dimensional cylindric algebras is not elementary. The construction we use when α is infinite also settles a conjecture of Tarski in the affirmative.