A 12-inch high-pressure natural gas pipeline exploded in Philadelphia Tuesday evening, killing a utility worker and injuring five others, including a firefighter. The cause of the explosion is not yet known.

With over 2 million miles of natural gas pipelines traversing the United States, natural gas explosions occur frequently. According to the Federal Pipeline and Hazardous Materials Safety Administration, there have been over 2,800 significant gas pipeline incidents since 1990, which together have injured or killed over 1,000 people.

In 2008 a natural gas explosion in Plum, Penn., seriously injured a four-year-old girl and killed a sixty-four-year old man when a two-inch natural gas distribution line leaked and exploded. Two homes were also destroyed and 11 others were damaged.

Last month, as part of our Watch List series of investigative reports, Need to Know examined the state of our nation’s natural gas pipeline system:

Watch the full episode. See more Need To Know.