  $A_2$Cr$_3$As$_3$ ($A$ = K, Rb, Cs) are the unique Cr-based ambient-pressure
superconductors to date discovered by serendipity in 2015. The new
superconducting family are structurally characterized by quasi one-dimensional
[(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ double-walled subnanotubes, which exhibit
peculiar properties that mostly point to unconventional superconductivity. In
this conference paper, we first describe how the superconductors were
discovered. Then we overview the recent progresses on crystal structure,
electronic structures, theoretical models, and various physical properties in
$A_2$Cr$_3$As$_3$. Some new experimental results are included. Finally we
conclude by addressing the related open questions in this emerging subfield of
superconductivity.
