Underground tunnels are a familiar feature in many of the world's major cities and they offer a fascinating lens through which to view their city's history, geography, and culture. One such city is Washington, D.C., which features an extensive tunnel system dating back to the mid-1800s. The D.C. Underground Atlas provides its visitors with an interactive digital guide to this tunnel system, much of which is physically inaccessible to the public. This series of engaging StoryMaps, accompanied by several articles and numerous photographs, allows visitors to virtually explore Washington, D.C.'s underground networks of transportation, utility, and pedestrian tunnels, and learn about their role in the city's history. The atlas' information was largely sourced from publicly available utility maps, newspaper archives, and government documents. The D.C. Underground Atlas was created in 2018 by Elliot Carter, an amateur cartographer who writes about D.C.'s hidden history for Atlas Obscura, and is supported by a grant from the DC Commission on the Arts and Humanities.