Dan Clarke pulled a large box from a shed in his backyard. He reached inside and removed a bag holding a black rock the size of a softball. "I found this one while gardening," he said, as he placed the big rock on the table. It crumbled slightly.

For the past five years, Clarke has amassed a collection of similar black rocks from his backyard in San Francisco's Marina district. Some are big and shiny, and others, small and dull. Clarke has found all of the black rocks while gardening, with the exception of the one set of rocks he discovered unexpectedly underneath his home when a sewer pipe burst in his garage. "After that moment," he said, "we realized it was everywhere."

The black rocks that Clarke has found beneath and around his home are made of coal and coal tar, a legacy of old manufactured gas plants (MGPs) that operated more than a century ago and powered homes and businesses throughout the Bay Area. MGPs converted dirty coal (or sometimes oil or wood) into gas that was then used for heating, lighting, and cooking. Although MGPs provided a vital service, they generated a slew of waste now known to be hazardous to humans and marine life. The waste, including the coal tar found beneath Clarke's home, is now a part of the city's — and the rest of the region's — ecosystem.

The concern over the discovery of the black rocks has moved people to take action and address the potential negative effects of living on top of a former industrial dump and working near it. Clarke, who has lived at his home on North Point Street since 2000, and the San Francisco Herring Association filed a lawsuit last year against the gas plant's owners, Pacific Gas & Electric Company, alleging that the byproducts created by three old San Francisco gas plants — Beach Street in the Fisherman's Wharf area and the Fillmore Street and North Beach facilities in the city's Marina district — are not only endangering human health, but also the herring that spawn in the bay.

PG&E officials declined to be interviewed for this story, but in an emailed statement, company officials said the utility is abiding by state environmental regulations.

However, Clarke and herring fishermen contend that PG&E has no real interest in solving the toxic problems that gas plants created years ago. "This needs to be addressed," said John Mellor, a herring fisherman and vice president of the San Francisco Herring Association. "It can't be swept under the table."

Clarke and the fishermen are not asking for monetary damages — rather, they want PG&E to conduct more extensive testing and clean up the polluted sites before the toxic waste spreads further.

MGPs represented a big leap in convenience and comfort for city dwellers between the mid 1800s and early 1900s. Initially invented to light street lamps, MGPs soon advanced to provide a constant source of energy for homes and businesses. Gas plants did this by turning solid coal into gas. The process began by heating coal in an oxygen-poor oven and waiting for it to gasify. The resulting gases contained a mix of toxic chemicals — including coal tar, residual coal, hydrogen sulfide, and ammoniac liquor — that the gas plant owners needed to remove before pumping the gas to residential and commercial buildings. After extracting the chemicals, gas plant owners sold some of them, including lampblack that was used for dyes, ink, tires, and other goods. Gas plant owners then buried the rest of the waste or dumped it into nearby waters.

click to enlarge Nadine Sebai

Dan Clarke found this coal rock and many others near his Marina district home.

The first commercial gas plant providing light for residents, streets, and businesses was in Baltimore in 1816. Small groups of local entrepreneurs soon began developing other gas plants in New York City and Philadelphia. As demand grew, gas entrepreneurs made their way west. Town after town adopted the new technology, until MGPs eventually arrived on the West Coast. By the mid-nineteenth century, almost every town in United States with a population of more than 10,000 had MGPs. After the discovery of gold in the Sierra Nevada, San Francisco's population increased 200-fold in a span of seven years, rising from about 200 residents in 1846 to about 40,000 in 1852. That same year, three brothers from Newark, New Jersey built one of the first MGPs in San Francisco.

Originally tradesmen who worked in the iron industry, the brothers — Peter, James, and Michael Donahue — opened and operated many MGPs in San Francisco, including the North Beach facility on North Point Street near Clarke's Marina district home. (The North Beach facility was in what is now known as the Marina district.) But ownership of MGPs changed hands often. In October 1905, PG&E formed and became the last owner of the North Beach, Fillmore Street, and Beach Street facilities. Six months after the utility acquired the North Beach and Fillmore Street facilities, the Great Earthquake of 1906 destroyed those plants, and PG&E abandoned them.

The 1906 earthquake also caused severe damage to the rest of the Marina district, an area where the city planned to construct the Pan Pacific International Exposition. The exposition, originally meant to celebrate the completion of the Panama Canal, instead became an opportunity for the city to showcase its recovery from the earthquake. This was especially apparent between April and September of 1912, when workers quickly filled in Marina Cove, a body of water next to the Marina district, with sand and sludge to make room for temporary exposition buildings.