acknowledge that such problems are more likely in places where industrial activity has changed the underground environment. There are upwards of 2 million abandoned and plugged oil and gas wells in the U.S., more than 100,000 of which may not appear in regulators' records. Sometimes they are just broken off tubes of steel, buried or sticking out of the ground. Many are supposed to be sealed shut with cement, but studies show that cement breaks down over time, allowing seepage up the well structure. Also, if injected waste reaches the bottom of old wells, it can quickly be driven back toward aquifers, as it was in Chico. "The United States looks like a pin cushion," said Bruce Kobelski, a geologist who has been with the agency's underground injection program since 1986. Kobelski spoke to ProPublica in May, 2011, before the EPA declined additional interview requests for this story. "Unfortunately there are cases where someone missed a well or a well wasn't indicated. It could have been a well from the turn of the [20th] century." Clefts left after the earth is cracked open to frack for oil and gas also can connect abandoned wells and waste injection zones. How far these man-made fissures go is still the subject of research and debate, but in some cases they have reached as much as a half-mile, even intersecting fractures from neighboring wells. When injection wells intersect with fracked wells and abandoned wells, the combined effect is that many of the natural protections