  Bulk rhombohedral Bismuth (Bi) at ambient pressure is a semimetal and it
remains in the normal state down to 10~mK. The superconductivity (SC) in bulk
Bi is thought to be very unlikely due to extremely low carrier density. The
question of SC in Bi has remained unsolved both theoretically and
experimentally. Here, we report first ever observation of bulk SC in highly
pure Bi single crystals (99.9999\%) below 0.53~mK under ambient pressure with
an estimated critical magnetic field of 5.2~$\mu$T at 0~K. The conventional
Bardeen-Cooper-Schrieffer (BCS) theory cannot explain the observed SC in Bi,
since the adiabatic approximation of the BCS theory, $\omega_D/E_F<< 1$, does
not hold true for Bi. Bi has a multi-valley type electronic band structure and
SC in Bi could be brought about by the inter-valley electron-phonon coupling.
Such a scenario calls for new theoretical ideas to understand SC in such low
carrier systems with unusual band structure in the non-adiabatic limit,
$\omega_D/E_F \gtrsim 1$. The observation of SC in Bi makes it the lowest
carrier density superconductor surpassing the record held by doped SrTiO$_3$
for nearly 50 years.
