We exposed standard Mo/Si multilayer coatings, optimized for 13.5 nm radiation to the intense femtosecond XUV radiation at the FLASH free electron laser facility at intensities below and above the multilayer ablation threshold. The interaction process was studied in-situ with reflectometry and time resolved optical microscopy, and ex-situ with optical microscopy (Nomarski), atomic force microscopy and high resolution transmission electron microscopy. From analysis of the size of the observed craters as a function of the pulse energy the threshold for irreversible damage of the multilayer could be determined to be 45 mJ/cm². The damage occurs on a longer time scale than the XUV pulse and even above the damage threshold XUV reflectance has been observed showing no measurable loss up to a power density of 10¹³ W/cm². A first explanation of the physics mechanism leading to damage is given.