For several decades now, microbiologists have lived with the growing realization that the majority of extant microbes are not in our culture collections. In fact, our historical reliance on cultivation to identify and quantify microbes has resulted in our missing upwards of 95% of extant bacterial and archaeal diversity. Using cultivation-independent molecular approaches to identify microbes by genetic sequence - specifically small subunit ribosomal RNA (SSU rRNA) sequences - we have begun to map the true microbial diversity of the Earth. This cultivation-independent approach to identifying diversity has recently benefited from the development of next-generation sequencing technology and a concomitant drop in sequencing costs. With respect to molecular surveys of microbial diversity in natural environmental samples, pyrosequencing approaches provide unprecedented sampling depth. Such deep sequencing has purportedly uncovered a rare and extensive biosphere of bacteria and archaea with a diversity that is perhaps several orders of magnitude greater than we had anticipated [1]. And although there may have been initial overestimations of the magnitude of the 'rare biosphere' because of the intrinsic sequence error rate pyrosequencing produces [2], many rare and novel microbes are still being discovered at taxonomic levels ranging from phyla to species.