  The study of the extreme weather space events is important for a
technological dependent society. Extreme Value Theory could be decisive to
characterize those extreme events in order to have the knowledge to make
decisions in technological, economic and social matters, in all fields with
possible impacts. In this work, the hourly values of the Dst geomagnetic index
has been studied for the period 1957-2014 using the peaks-over-threshold
technique. The shape parameter obtained from the fit of the generalized Pareto
distribution to the extreme values of the |Dst| index leads to a negative value
implying an upper bound for this time series. This result is relevant, because
the estimation of this limit for the extreme values lead to 850 nT as the
highest expected value for this geomagnetic index. Thus, from the previous
characterization of the Carrington geomagnetic storm and our results, it could
be considered the worst case scenario.
