A substantial amount of evidence has accumulated over the past decade to indicate the presence of a deep sub-seafloor microbial biosphere on Earth. Microbial communities have now been identified within hydrothermal vent systems at ocean floor spreading centers, deep within oceanic sediments, and at depth in the volcanic basement of young oceanic crust. Questions remain as to how long such microbiological activity persists within the oceanic crust and how well it may be preserved in the geological record. Here we report chemical, isotopic, and new morphological evidence of relic microbial activity in the glass rims of 110 Ma pillow basalts at DSDP Hole 418A in the North Atlantic Ocean (to a depth of 862 m below the seafloor, or 538 m below the top of the volcanic basement). X-ray mapping and point analyses (via electron microprobe) were carried out on basaltic glass samples from Hole 418A displaying patchy, irregular alteration textures adjacent to fractures. The patchy alteration fronts were found to contain significantly high concentrations of C (up to 31.7 wt%), that correspond with depleted values of Ca, Fe, and Mg with respect to the fresh glass. Stable carbon and oxygen isotope studies of basaltic glass samples, also indicates that the textures could have been produced via biogenic processes. The low delta carbon-13 values determined from disseminated carbonate (e.g.: -12.8 per mil (PDB)) represent biologically fractionated carbon, and respective delta oxygen-18 values (e.g.: 15 to 30 per mil (SMOW)) are consistent with a 15 to 110 degrees C environment for the microbes (which is well within the temperature conditions necessary to harbor life). SEM imaging of unaltered basaltic glass chip samples (DSDP sample: 418A-75-3-(120-123)) has revealed the presence of isolated networks of anastomosing nannoscopic tunnels. The tunnels are typically 150-200 nm across and are commonly infilled with pellets ( approximately 30-40 nm in diameter), filaments (20 nmX100 nm), and structures bearing resemblance to a dessicated polysaccharide mucus. Morphological and textural observations indicate that the tunnels may represent preserved nannoscopic trace fossils left behind by sub-micron-sized organisms.