Trends in microbiology has a minireview about bacterial cells growing as deep as 800 meters underneath the seafloor. The numbers of bacteria, and the size of marine sediments, means that these prokaryotic communities might be as much as 30% of the total earth biomass.
The main focus of the minireview is on efforts to count how many are actually alive. Early counts based on sulfur turnover gave values orders of magnitude smaller than surface counts, which suggested that a lot of the biomass down there was either dead or very metabolically slow. A new effort used an alternative technology (detecting ribosomal RNA) and came up instead with fairly constant numbers of alive cells from the surface all the way down. So the bacteria are dividing slowly, but they are alive and well.
800 meters underground!