The U.S. Department of Homeland Security (DHS) Science and Technology Directorate (S&T), through its Resilient Tunnel Plug project, is developing technologies that prevent or limit flooding in subway tunnels. In March 2017, S&T successfully completed three full-scale validation tests of the tunnel plug in a test tunnel designed to closely replicate an actual subway tunnel. Following plug deployment and inflation, the test tunnel was flooded and pressurized to simulate real-world flooding conditions. The tests evaluated performance of the plug itself, storage container, inflation pumps, and control systems. A “long duration” test, where water pressure was continuously applied for 21 days to mimic an extended flooding event, was also successful.