Cover photograph (Copyright © 2007, American Society for Microbiology. All Rights Reserved.): Thick white microbial biofilms carpeting stream channels within the Frasassi cave system, Italy. The cave system is actively forming by sulfuric acid speleogenesis, a microbially mediated process responsible for some of the largest caves known. Microbial sulfuric acid production at Frasassi has carved >25 km of cave passages over the course of more than 200,000 years. The biofilms are dominated by filamentous sulfur-oxidizing Beggiatoa spp., in association with diverse sulfur-reducing bacteria, hereotrophs, and other sulfur oxidizers. Chemoautotrophic primary production in the biofilms supports a subsurface ecosystem including protists and macroinvertebrates. Image width, ∼2 m. (See related article in the August 2006 issue: volume 72, page 5596.)