The Unit Episode 4.16 Hill 60

The Unit and their families are scattered around their community when chlorine gas leaks into the water system, poisoning everyone who is outside and cutting off communication with the outside world. Tiffy, who is supervising detention at the school, manages to seal off the classroom and protect the students with her. Grey is on the road, but secures a number of other drivers in the back of a closed refrigerator truck. Mack, his kids, Molly, Kim and Bob, make it back to their apartment building where they aid other residents to safety. Mack and Bob find a neighbor's scuba equipment and use it to make it to the Aerodyne offices, where they find Jonas and the new recruit, Sam. Jonas and Sam are the only ones with outside communication -- they've located the where the chlorine gas had to be inserted in the system and make ready to remedy the problem and deal with any potential terrorists (it's unclear whether its a massive accident, or terrorist activity).