First you must unlock the Kill Event called "Rolling Thunder," which appears between the East Village and the Lower East Side (one block from the East coast). Right next to the Event is a Thermobaric Tank. Jumping into it will activate the Event, but you can back out of it before the Event begins. Do this correctly and you'll have a free tank. Note: If you exit the tank it or use it to destroy a Hive or Base it will vanish. You can grab it again at the Event point.