  We report on the X-ray properties of the exceptional X-ray cluster
RXJ1347.5-1145 at z=0.451. We confirm that it is with a luminosity
$L_X(bol)=2\times 10^{46}$ erg/s the most luminous X-ray cluster discovered to
date. The mass of the cluster within 1.7 Mpc is $9.8%\pm1.1\times 10^{14}
\msol$. A comparison of the central X-ray mass and the mass determined from a
simple gravitational lens model shows a discrepancy of a factor of 2-3 with the
X-ray mass being smaller. The temperature of the cluster is
$9.3^{+1.1}_{-1.0}$~keV. We detect a strong FeK line corresponding to a
metallicity of $0.33\pm0.10$ in solar units, which is an unexpectedly high
value for a distant and hot cluster. There are several hints that the cluster
contains an extremely strong cooling flow. With the usual assumptions we derive
formally a mass accretion rate of more than 3000 $\msol$/yr indicating that
this may be the largest cooling flow detected so far. To find these extreme
properties in this distant cluster which can be taken as an indication of a
well relaxed and old system is of high importance for the theory of formation
and evolution of clusters.
