  It was presumed that semimetal Bismuth (Bi) would not show superconductivity
(SC) even at ultra-low temperatures ($<$10 mK) due to its very low carrier
density ($\approx 3\times10^{17}$cm$^{-3}$). Recently, we have established bulk
superconductivity in ultra-pure (99.9999\%) Bi single crystal at $\mathrm{T_C =
0.53}$ mK with an extrapolated upper critical field $\mathrm{H_C(0) = 5.2\mu}$T
measured along the [$0001$] (trigonal) -crystallographic direction. At very low
concentrations of the charge carriers, we are dealing with fragile Cooper pairs
with an estimated large coherence length $\mathrm{\xi_{GL}(0)\approx 96 \mu}$m.
We also stated that one needs to go beyond the conventional electron-phonon
coupling (BCS-like) mechanism to understand the SC state in Bi. Bi is a
compensated semi-metal with electrons and holes as charge carriers. In order to
find the charge carriers responsible for the SC, we report the temperature
dependence of the anisotropic critical field along the [$01\bar 10$]
(bisectrix)-crystallographic direction and compared it with the earlier data
from measurements along the trigonal. Our theoretical analysis of the
anisotropy of critical fields suggests that the light electrons in the three
pockets of Bi bands are responsible for the SC and indicates that Bi is an
extremely weak type-II (close to type-I) superconductor. Finally, we review the
current theories proposed to explain the SC in Bi.
