Twenty years ago in Chicago, a small leak in an unused freight tunnel expanded beneath the Windy City and started a flood which eventually gushed through the entire tunnel system. A quarter-million people were evacuated from the buildings above, nearly $2 billion in damages accrued, and it took 6 weeks to pump the tunnels dry. How much more costly in lives and infrastructure would a flood in a heavily used, underwater subway tunnel be today? In January 2012 the Department of Homeland Security (DHS) Science and Technology Directorate (S&T) successfully tested an unprecedented technology for containing flooding or dangerous gases in mass transit tunnels: a giant plug. S&T’s Resilient Tunnel Project (RTP) has developed an enormous inflatable cylinder, tunnel-shaped with rounded capsule-like ends, that can be filled with water or air in minutes to seal off a section of tunnel before flooding gets out of control. S&T’s new tunnel plug provides an affordable, easily installed, quickly deployable solution to protect vital mass transit systems. Developed in partnership with Pacific Northwest National Laboratory, West Virginia University (WVU) and ILC Doverlongtime maker of NASA space suitsthe plug inflates (with water or air) to dimensions of roughly 32-feet-long and by 16-feet-wide, and holds 35,000 gallons, about the same capacity as a medium-sized backyard swimming pool. When not in use, the plug packs down to a small storage space in the tunnel, ready for remote, immediate inflation in an emergency from the tunnel system’s command center.