  The recent discovery of superconductivity at 190~K in highly compressed
H$_{2}$S is spectacular not only because it sets a record high critical
temperature, but because it does so in a material that appears to be, and we
argue here that it is, a conventional strong-coupling BCS superconductor.
Intriguingly, superconductivity in the observed pressure and temperature range
was predicted theoretically in a similar compound H$_{3}$S. Several important
questions about this remarkable result, however, are left unanswered: (1) Does
the stoichiometry of the superconducting compound differ from the nominal
composition, and could it be the predicted H$_{3}$S compound? (2) Is the
physical origin of the anomalously high critical temperature related only to
the high H phonon frequencies, or does strong electron-ion coupling play a
role? We show that at experimentally relevant pressures H$_2$S is unstable,
decomposing into H$_3$S and S, and that H$_3$S has a record high $T_c$ due to
its covalent bonds driven metallic. The main reason for this extraordinarily
high $T_c$ in H$_3$S as compared with MgB$_2$, another compound with a similar
superconductivity mechanism, is the high vibrational frequency of the much
lighter H atoms.
