By Greg Argos

PHILADELPHIA (CBS) — There’s a secret place just south of Philadelphia. Many years ago it was the site of a dinner party some 300 feet underground, but that’s not where the story ends.

At the entrance to the Marcus Hook Industrial Complex sits a bronze statue of a sun oil seaman looking out to sea.

“Marcus Hook was an important part of the war effort during World War II,” said Jonathan Hunt, senior director of Marcus Hook Industrial Complex.

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It memorializes the 141 mariners killed after German U-boats torpedoed oil tankers leaving the facility.

The facility no longer refines oil. It’s now used primarily to store natural gas and its by products. And it’s one of the largest natural gas storage facilities in the Northeast.

“All told, it’s roughly 5 million barrels of natural gas liquid storage,” Hunt said.

But what makes this sprawling 800-acre facility so unique isn’t the towering storage tanks dotting the property, but what sits hundreds of feet below.

“The largest cavern here on Marcus Hook is a propane cavern which we store approximately 1 million barrels of propane,” said Hunt.

In the 1950s, the former owner discovered a large chunk of the facility sits on solid granite. So digging began and five caverns were built, the floors of each between 300 and 500 feet below ground.

Pictures show their immense size before they were filled with natural gas. A banquet dinner for dozens was once held on the cavern’s floor.

The caverns that were built here were part of really a response to what was happening during the Cold War and trying to come up with ways to store products safely with the threat of the Cold War.

Even today, security is tight and the entrances to the caverns are unmarked.

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“This facility is regulated under the Maritime Safety and Security Act so we work very closely with the Coast Guard and government security to ensure we provide a secure site as well from any external threats,” said Hunt.

Fortunately, there haven’t been any and the facility had transformed with the times. From a crude oil refinery supplying the wartime effort, to a natural gas supply hub where tractor trailers, trains and tankers fill up and move out.

“This site has been an important part of energy use, production and distribution in the Northeast since the turn of the century in the early 1900s. So with renaissance in gas and natural gas liquid production here in Pennsylvania,” Hunt said.

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