"All evidence points to oil in the Cupertino basin, particularly trapped along fault zones and along the margins [of the basin]," said Richard Stanley, a researcher with the U.S. Geological Survey.

Stanley first discovered the basin oil after coming across an old oil pump in the parking lot of a commercial building in Los Gatos. "It piqued my interest and we took geological samples," he said of the findings first reported by the survey in 2002. Stanley explained the geology of the basin had previously been poorly understood, and by investigating the source of the oil, researchers unearthed an underlying layer of sedimentary rocks known as Miocene. And as a result, "we found oil buried about 2.5 kilometers deep," he said.

The subsurface oil in fact sparked a now largely forgotten mini-oil boom in the Santa Clara Valley from the 19th century to the 1930s, Stanley said. The Los Gatos pump reportedly lasted longer, pumping an estimated eight barrels a day until it ceased operation in 1953. According to the National Park Service, Santa Clara County produced all of California's oil from the 1860s to 1880, with oil wells dotting the valley during that time.

Before breaking out drills and surveying equipment, however, future wildcatters are forewarned such boom times are long over.

"There is probably not enough to be of commercial interest," Stanley said. The highly populated, urbanized Cupertino basin also likely precludes any drilling. The seismically active Cupertino basin stretches from Palo Alto southwest to Los Gatos, bordering the Santa Cruz Mountains in the west and Sunnyvale to the east.

Stanley said the oil poses no major hazards but did say, "It would be wise to drill well bores carefully, in case any oil moves up [the line] and contaminates water supplies. And if people find traces of oil in their water, it is likely from the oil underground—not man-made pollution."