This is Sunoco’s Mariner East pipeline system, one of the most controversial projects in recent Pennsylvania history.

The system stretches more than 350 miles, from Ohio and Southwestern Pennsylvania to Sunoco’s refinery in Marcus Hook, just outside Philadelphia.

Mariner East pumps natural gas liquids, and because of the fracking boom, there is a need to transport more.

The chemicals are pushed through several lines at high pressure — thousands of times the force typically used to send gas to a kitchen stove.

Because of the type of chemicals moving through the pipeline, a leak could lead to a big problem in a short amount of time.

But natural gas liquids don’t leak as liquids. They immediately vaporize, forming a cloud that hangs close to the ground.

The clouds are odorless, colorless, and highly flammable. They could asphyxiate anyone who walks into them.

While the risk of an accident is low, the dangers of natural gas liquids pose a unique challenge for emergency planners.

Mariner East runs through densely populated areas, and many residents say they don’t know what to do if a serious accident occurs.

Based on a study commissioned by a group of residents and townships in suburban Philadelphia, and Sunoco’s estimates for a smaller pipeline in Canada, an explosion could pose a risk to people within roughly half a mile.

Another estimate, commissioned by Delaware County, found the impact could span roughly 1.3 miles.

Roughly 345,000 Pennsylvanians live within that distance of the pipeline, according to an analysis of U.S. Census data.

More than 300 vulnerable buildings — like schools, hospitals, and churches — that house young children, the elderly, and the disabled are also within that area.

But the exact risk remains unknown, because a state law allows Sunoco to keep that and other information about the pipeline a secret.

Without that knowledge, planning for an accident — and how to safely escape — is very difficult, first responders and emergency managers say.

The company says the public has enough information, and it has sent 324,000 informational brochures to residents and trained just over 2,000 first responders.

But state and federal regulators disagree. And the safety track record of Sunoco and its parent company, Energy Transfer Partners, makes critics more concerned.

Together, the companies have tallied dozens of citations from regulators for issues including contaminating drinking water, causing sinkholes in backyards and roads, and other failures.

It’s left many asking: Is Pennsylvania prepared if something goes wrong?