The ultrashort pulse laser writing multi-layered data bits in fused silica for three-dimensional optical data storage through the mechanism of optical damage induced by multi-photon absorption is demonstrated. The 800 nm, 120 fs Ti:Sapphire laser beam is focused into cubic samples with a 0.65 NA commercial microscope objective. The size of the recorded bits can be smaller than 0.5×0.5×2.5 µm as the laser pulse energy is lower than 300 nJ and the corresponding bit data storage density is 0.2×10¹² Tbits/cm³. Due to the aberration produced by the mismatch of the refractive indices between the recording material and its immersion medium, the longitudinal size of the recorded bit increases and the contrast of the bit to the surrounding uninfected zone decreases as the bit plane gets deeper. Hence, a relatively lower storage density of 0.05×10¹² Tbits/cm³ is realized.