Peculiar properties of Cr$_3$As$_3$-chain-based superconductors
$A_2$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.