  The Hubble parameter $H_0$, is not a univocally-defined quantity: it relates
redshifts to distances in the near Universe, but is also a key parameter of the
$\Lambda$CDM standard cosmological model. As such, $H_0$ affects several
physical processes at different cosmic epochs, and multiple observables. We
have counted more than a dozen $H_0$'s which are expected to agree if a) there
are no significant systematics in the data and their interpretation and b) the
adopted cosmological model is correct.
  With few exceptions (proverbially confirming the rule) these determinations
do not agree at high statistical significance; their values cluster around two
camps: the low (68 km/s/Mpc) and high (73 km/s/Mpc) camp. It appears to be a
matter of anchors: the shape of the Universe expansion history agrees with the
model, it is the normalizations that disagree.
  Beyond systematics in the data/analysis, if the model is incorrect there are
only two viable ways to "fix" it: by changing the early time ($z\gtrsim 1100$)
physics and thus the early time normalization, or by a global modification,
possibly touching the model's fundamental assumptions (e.g., homogeneity,
isotropy, gravity). None of these three options has the consensus of the
community.
  The research community has been actively looking for deviations from
$\Lambda$CDM for two decades; the one we might have found makes us wish we
could put the genie back in the bottle.
