RX J1347-1145 is an extreme galaxy cluster discovered in the ROSAT All-Sky Survey. Follow-up observations in optical and X-rays (with ROSAT/HRI and ASCA) reveal a number of interesting properties. The image shows HRI contours superposed on an NTT image, displaying a very compact cluster. In the optical image several arcs and arc candidates are visible (marked with letters), implying that the cluster acts as a gravitational lens. RX J1347-1145 at z = 0.451 with a luminosity L_X(bol) = 2 x 10⁴⁶ erg/s is the most luminous X-ray cluster discovered to date. The cluster has a strong cooling flow of more than 3000 solar masses per year, the largest cooling flow detected so far.