  The Aria project consists of a plant, hosting a 350 m cryogenic isotopic
distillation column, the tallest ever built, which is currently in the
installation phase in a mine shaft at Carbosulcis S.p.A., Nuraxi-Figus (SU),
Italy. Aria is one of the pillars of the argon dark-matter search experimental
program, lead by the Global Argon Dark Matter Collaboration. Aria was designed
to reduce the isotopic abundance of $^{39}$Ar, a $\beta$-emitter of cosmogenic
origin, whose activity poses background and pile-up concerns in the detectors,
in the argon used for the dark-matter searches, the so-called Underground Argon
(UAr). In this paper, we discuss the requirements, design, construction, tests,
and projected performance of the plant for the isotopic cryogenic distillation
of argon. We also present the successful results of isotopic cryogenic
distillation of nitrogen with a prototype plant, operating the column at total
reflux.
