Go long. Hear from all the gas pipes, hissing and spewing flammable gas over the towns. So, we wondered, exactly how much of that gas is needed to cause an explosion? And abc's senior national... See More
Go long. Hear from all the gas pipes, hissing and spewing flammable gas over the towns. So, we wondered, exactly how much of that gas is needed to cause an explosion? And abc's senior national correspondent jim avila set out to show us tonight. Jim? Reporter: So, diane, we're at the virginia beach, virginia, training center, I'm with a firefighter here who is going to show us what happened when a commercial gas meter breaks. The pilot light will be on. And look how fast it will take off. It's a frightening sound, and sight. Natural gas burns fast and long. And after a natural disaster like hurricane sandy, where houses have been moved off their foundations, and whole blocks levelled from fire, it's a hazard at every turn. There is little clue, except a sulfur smell, that is purposely added to the gas as a warning that danger lingers. So what are we approaching here? You can kind of smell that rotten egg smell. Right. We're simulating a broken gas line here, and that smell is the merkapten injected into natural gas. Reporter: Okay, so when it goes up, end kind of goes up like your outside gas grill, it's hot. Right. Right now it's seeking an ignition source. Ignition source can come from anything from a fuse on a light pole or a cigarette butt. Reporter: But there's no real explosion, right? No, once the gas is seek, 5% to 17% flammable range and bam, it goes off. Inside a structure, that's where you're going to get your explosion. Reporter: There's plenty of evidence of that. Watch as this house is rocked during a training exercise by arson investigators. The explosions are very hazardous, they can generate pressures on the order of 35 atmospheres. And these are like a military ballistic blast that can level houses and kill people within a great distance. Reporter: And while our demonstrations today used flares to ignite, sources of ignitions are everywhere in a disaster zone. Virtually anything can be an ignition source. Molt or thes or fans can ignite it or heat, the temperature, something that's cherry red, an electrician stove that's cherry red is hot enough to ignite the mixture. Reporter: Diane, that's the sound that people all over new jersey are hearing. The thing the fire department says here is, don't panic, because it takes direct ignition to get it started if you're outside. If you're inside a house and that happens, get out right away, then call 911.
This transcript has been automatically generated and may not be 100% accurate.