The Resilient Tunnel Project (RTP) is a giant inflatable plug designed to contain flooding or dangerous gases in mass transit tunnels. Developed by the US Department of Homeland Security’s Science and Technology Directorate, the RTP is a 35,000 gallons (about the same capacity as a medium-sized backyard swimming pool) tunnel-shaped plug 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.
From BLDGBLOG