Hundreds of feet beneath the former Sunoco refinery in Marcus Hook are five caverns carved from granite, excavated not by ancient geologic forces but by 20th-century miners.
The first of the gigantic underground fuel storage caverns was built during the Cold War, partly to shield supplies from nuclear attack. The last was built after the 1973 energy crisis as a bulwark against capricious oil importers.
Now, decades later, the national debate focuses on American energy exports, not imports, and the caverns have assumed new importance in response to the nation's changing energy fortunes. The caverns, which hold 2 million barrels, are the largest underground fuel storage facility on the East Coast.
Sunoco Logistics Partners L.P., which took over the refinery after it shut down in 2011, is rebuilding the Marcus Hook site as a hub for exporting Marcellus Shale natural gas liquids. The caverns are uniquely suited for storing propane and butane until the materials can be exported.
Liquid fuels such as propane, butane, and ethane are extracted from natural gas and will be transported to Marcus Hook through the $3 billion Mariner East pipeline project under development.
The caverns appear in no tourist guide. "For most people around here, they're not even aware they're there," said Jonathan Hunt, director of the Marcus Hook Industrial Complex, the former refinery's new name.
Within Sunoco, the caverns are the stuff of lore.
A photograph from the 1950s hangs on a wall of Hunt's office, showing a celebratory luncheon in a 50-foot-high cavern that resembles a rough-hewn cathedral. Nobody has entered the caverns since construction was completed. Pumps and other equipment can be pulled up to the surface for maintenance. But there is no spelunking.
"It's theoretically possible," Hunt said. "But you'd have to remove all the fuel and then decontaminate it so it was safe to enter, which would be difficult."
There are no identification signs on the surface, just pumps, pipes, and tanks indistinguishable from the steel latticework on the 790-acre Marcus Hook site.
"People who come to Marcus Hook really want to see the caverns," Hunt said. Visitors see the wellheads, but they are invariably disappointed. "There's not a whole lot to see."
The Marcus Hook caverns were designed for propane and butane, which are used as fuel and as a raw material for petrochemicals and plastics. Butane is blended into gasoline during cold months to give the fuel a boost.
Construction of the first cavern in 1957 was hard to miss. Great cylinders of excavated granite were heaped along the roadside, awaiting disposal, according to news accounts.
A 1957 report by the National Petroleum Council advocated expansion of underground storage for national security reasons. Underground storage was typically built from excavated salt domes on the Gulf Coast. Hard-rock storage caverns were more unusual, and more expensive to build.
Each Marcus Hook cavern is accessed through a five-foot-wide shaft, through which the miners and the broken rock were lifted in and out. Mining equipment was broken down and reassembled in the cavern.
The caverns lie under about 10 acres of the refinery and are honeycombs of spaces separated by 40-foot-thick granite pillars to maintain overhead stability. The cavern floors are between 300 feet and 500 feet below the earth's surface. The granite roofs are hundreds of feet thick.
"A hydrogen bomb could hit within 50 feet of the cavern and cause no damage to it, whereas a bomb hit 20 miles away might topple surface tanks," Patrick E. Dougherty, a Sun engineer, told a reporter in 1958.
The last of the five caverns was built for $16 million in 1974 to store imported propane to replace shrinking domestic supplies of liquefied petroleum gas, or LPG.
"No matter how much we try to conserve fuel, there is going to be a significant growth in demand for LPG," Norman Sumner, a Sun Oil manager, said in 1974.
The caverns take advantage of natural subterranean pressure, which maintains propane and butane in a liquid state. At atmospheric pressure, propane and butane turn to vapor - think of a gas grill.
In 1978, faulty monitoring caused too much butane to be pumped into Cavern No. 3, and some leaked into basements of nearby rowhouses, said Louis V. Gonzalves Jr., operations manager of the Marcus Hook complex. Five houses on the 1100 block of Green caught fire. About 30 families were displaced for two months.
City Councilman Joseph Gattone called the caverns "a keg of dynamite." Curt Weldon, then Marcus Hook's mayor, called for closing the caverns.
But Sun said it changed its practices, the residents returned, and the memory of the caverns largely receded, except among a few long-term residents like Carmen Dickerson, 80. She remembers her mother took a while to recover from the fear.
"Mother slept on the couch with the house keys in her pocket for two years after the fire in case she needed to get out in a hurry," she said.
Sunoco sold the smallest of the caverns to Braskem USA, which acquired Sunoco's nearby polypropylene plant. The remaining caverns fit in snugly with its plans to build the Mariner East project.
The Mariner East pipelines have generated some opposition from residents who object to the export of Pennsylvania fuels. Though some propane is sold locally, most of the fuel is committed to overseas buyers.
When the second Mariner East pipeline is completed next year, Sunoco Logistics will move and process 345,000 barrels of natural gas liquids a day through Marcus Hook. Maritime officials expect 100 ships a year to load fuel at Marcus Hook.
Sumner, the Sun manager who supervised the cavern construction in 1974, says he is not surprised they were being repurposed for exports.
"It's illustrative about the energy industry," said Sumner, 80, now retired and living in Horsham. "It's vibrant, and always changing."