The rich cluster Abell 520 (z=0.201) exhibits truly extreme and puzzling multi-wavelength characteristics. It may best be described as a cosmic train wreck. It is a major merger showing abundant evidence for ram pressure stripping, with a clear offset in the gas distribution compared to the galaxies (as in the bullet cluster 1E 0657-558). However, the most striking feature is a massive dark core (721 h_70 M_sun/L_sun) in our weak lensing mass reconstruction. The core coincides with the central X-ray emission peak, but is largely devoid of galaxies. An unusually low mass to light ratio region lies 500 kpc to the east, and coincides with a shock feature visible in radio observations of the cluster. Although a displacement between the X-ray gas and the galaxy/dark matter distributions may be expected in a merger, a mass peak without galaxies cannot be easily explained within the current collisionless dark matter paradigm. Interestingly, the integrated gas mass fraction (~0.15), mass-to-light ratio (220 h_70 M_sun/L_sun), and position on the X-ray luminosity-temperature and mass-temperature relations are unremarkable. Thus gross properties and scaling relations are not always useful indicators of the dynamical state of clusters.
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