  As an important source for large geomagnetic storms, an "ICME-in-sheath" is a
completely shocked interplanetary coronal mass ejection (ICME) stuck in the
sheath between a shock and host ejecta. Typical characteristics are identified
from coordinated multi-sets of observations: (1) it is usually short in
duration and lasts a few hours at 1 AU; (2) its solar wind parameters, in
particular the magnetic field, seem to keep enhanced for a large range of
distances; and (3) common ICME signatures are often lost. The host ejecta could
be a single ICME or a complex ejecta, being fast enough to drive a shock. These
results clarify previous misinterpretations of this phenomenon as a normal part
of a sheath region. The "ICME-in-sheath" phenomenon, together with a
preconditioning effect, produced an extreme set of the magnetic field, speed
and density near 1 AU in the 2012 July 23 case, all around their upper limits
at the same time. This is probably the most extreme solar wind driving at 1 AU
and enables us to estimate the plausible upper limit for geomagnetic storm
activity. With an appropriate modification in the southward field, we suggest
that a geomagnetic storm with a minimum $D_{\rm st}$ of about $-2000$ nT could
occur in principle. The magnetopause would be compressed to about 3.3 Earth
radii from the Earth's center, well inside the geosynchronous orbit.
