Dark etching regions (DERs) are a widely studied phenomenon in the context of sub-surface microstructure decay in rolling bearings. These regions result from dislocation motion (plasticity) and carbon migration due to rolling contact fatigue (RCF). We conducted a systematic study using Mössbauer spectroscopy to identify phase evolution during tempering and DER formation. Our findings resolve a long-standing debate in the literature by demonstrating that fine temper carbides dissolve during DER formation. A slight decrease in non-stoichiometric carbides was observed in the DERs, indicating the dissolution of fine carbides. The released carbon is believed to back-diffuse into ferrite and/or martensite.