Soil biota play key roles in the functioning of terrestrial ecosystems, however, compared to our knowledge of aboveground plant and animal diversity, the biodiversity found in soils remains largely uncharacterized. Here we present an assessment of soil biodiversity and biogeographical patterns across Central Park in New York City that spanned all three domains of life, demonstrating that even an urban, managed system can harbor large amounts of undescribed soil biodiversity. Despite high variability across the Park, belowground diversity patterns were predictable based on soil characteristics, with prokaryotic and eukaryotic communities exhibiting overlapping biogeographical patterns. Further, Central Park soils harbored nearly as many distinct soil microbial taxa and types of soil communities as we found in biomes across the globe (including arctic, tropical and desert soils). This integrated cross-domain investigation highlights that the amount and patterning of novel and uncharacterized diversity at a single urban location matches that observed across natural ecosystems spanning multiple biomes and continents. Our data therefore suggest that the dominant factors controlling soil biodiversity differ markedly to those controlling aboveground plant and animal diversity.