Pennsylvania is trying to solve its orphaned well problem. But it's still trying to figure out just how many there are.

Pennsylvania is sitting on what many experts describe as one of the largest abandoned oil and gas well problems in the United States, and the full scale of it still is not completely understood.

State officials and researchers estimate Pennsylvania may contain anywhere from 300,000 to 700,000 abandoned or orphaned oil and gas wells, most of them concentrated in the western portion of the state. Many were drilled decades ago, long before modern environmental regulations existed. Some date back to the earliest days of America’s oil boom in the late 1800s.