  It is paramount from both scientific and societal perspectives to understand
the generation of extreme space weather. We discuss the formation of solar
superstorms based on a comparative study of the 2012 July 23 and 2017 July 23
eruptions. The first one is Carrington-class, and the second could rival the
1989 March event that caused the most intense geomagnetic storm of the space
age. Observations of these events in the historically weak solar cycle 24
indicate that a solar superstorm can occur in any solar cycle and at any phase
of the cycle. Recurrent patterns are identified in both cases, including the
long-lived eruptive nature of the active region, a complex event composed of
successive eruptions from the same active region, and in-transit interaction
between the successive eruptions resulting in exceptionally strong ejecta
magnetic fields at 1 AU. Each case also shows unique characteristics.
Preconditioning of the upstream solar wind leading to unusually high solar wind
speeds at 1 AU is observed in the first case whereas not in the latter. This
may suggest that the concept of "preconditioning" appears to be necessary for
making a Carrington-class storm. We find a considerable deflection by nearby
coronal holes in the second case but not in the first. On the basis of these
results, we propose a hypothesis for further investigation that superstorms are
"perfect storms" in nature, i.e., a combination of circumstances that results
in an event of unusual magnitude. Historical records of some extreme events
seem to support our hypothesis.
