Tunnels are integral to our nation’s transportation infra-structure. They provide main routes in and out of a region and can be an attractive target for any attack. Most domes-tic tunnels were not designed with explosives attacks in mind; however, a well-placed explosive device could have catastrophic results, with flooding as a major concern for underwater tunnels. The Department of Homeland Security (DHS) Science and Technology Directorate (S&T) conducted research to identify ways to reinforce our tunnels and shield them from potential threats. S&T’s Protective Measures and Design Tool project developed and tested flexible concrete mats that can be used to protect underwater tunnels.