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Microbiome profiling using the 16S rRNA gene was used to examine the composition and diversity of bacteria residing in thin, whitish, iridescent microbial biofilms 1470 m below the surface in the Sanford Underground Research Facility (SURF), a former gold mine in South Dakota, USA. The biofilms superficially resemble "cave silver" biofilms described from limestone caves. The communities are dominated by bacteria, although significant Thuamarchaeota are also present. The most abundant bacterial groups were Actinobacteria (mainly Pseudonocardiaceae), Acidobacteria, Proteobacteria, Chloroflexi, and Planctomycetes. The composition of the biofilms varied considerably from sample to sample. Like cave silver biofilms in Europe and other locations, Proteobacteria and Actinobacteria are abundant, but nearly all operational taxonomic units (OTUs) (at 97% similarity) are different. Apart from one Pseudonocardia species, most abundant OTUs observed here were not isolated by a previous culture-based study at SURF. This indicates that culture-based techniques do not accurately represent the diversity of bacterial taxa in SURF "cave silver." [ABSTRACT FROM AUTHOR]