CHEYENNE, Wyo. (AP) — Peabody Energy’s bankruptcy filing helps draw attention to plans for financially troubled coal companies to cover the potential cost of filling in mines that close.

Peabody’s mines include the top-producing coal mine in the U.S., the huge North Antelope Rochelle mine in Wyoming’s Powder River Basin.

St. Louis-based Peabody filed for Chapter 11 bankruptcy Wednesday. Environmental groups warn the recent bankruptcy of several major coal companies could leave taxpayers responsible for billions in reclamation costs should coal mines start closing nationwide.

A key issue is a practice called self-bonding. Self-bonding allows coal companies promise to pay for mine cleanup instead of posting bond for mine reclamation up front.

Peabody has more than $1 billion in self-bonding obligations in Wyoming, Illinois, Indiana, Colorado and New Mexico.