Along the base of Little Bahama Bank and out into the northeastern Straits of Florida in depths of 600 to 700 m, the research submersible Alvin encountered an extensive area of rocky mounds that are hundreds of metres long and as high as 50 m. The larger mounds are elongated in the direction of northerly bottom flow. They are composed of surface-hardened concentric crusts of submarine-lithified muddy to sandy carbonate sediment upon which a dense and diverse community of benthic organisms such as crinoids, corals, and sponges attach. Many of the organisms grow oriented into the prevailing northward current. The crusts are cemented in varying degree by micritic magnesian calcite and are intensively bored by endolithic organisms. Excavation of the softer undercrust by currents and (or) organisms can produce stromatactoid voids. It appears that these deep mounds are biohermal in nature and constructed in situ by the subsea lithification of successive layers of trapped sediment and deposited skeletal debris. The term “lithoherm” is proposed to describe the mounds. These and the hardgrounds between them indicate that subsea lithification aids in the accretion of the flanks of carbonate platforms. Micritic carbonate build-ups similar to lithoherms have been recognized in the geologic record. The mounds of the Waulsortian bank margin offer one possible ancient analog.