The Urban Cryptobiotic Crust: Many of the roofs in our towns and cities have become covered in a dark patina of microbial growth. It might not look alive, but were we able to observe this thin layer with a microscope we would find an exotic and miniature forest inhabited by fungi, green algae and cyanobacteria. I call this ubiquitous but overlooked microbiological veneer, the Urban Cryptobiotic Crust (UCC).  Here the effluent from two fans, and the wash from a lead flashing, has prevented the growth of the UCC, and by causing visible zones of inhibition,  this has highlighted this overlooked ecology.