Chesapeake Energy Corp,the Oklahoma-based firm is the No. 1 driller in Ohio.
Rig Count Interactive Map by Baker Hughes, an energy services company.
Shale Sheet Fracking, a Youngstown Vindicator blog.
The Ohio Environmental Council, a statewide eco-group based in Columbus.
Earthjustice, a national eco-group.
People's Oil and Gas Collaborative-Ohio, a grass-roots group in Northeast Ohio.
Concerned Citizens of Medina County, a grass-roots group.
No Frack Ohio, a Columbus-based grass-roots group.
Fracking: Gas Drilling's Environmental Threat by ProPublica, an online journalism site.
Pipeline, blog from Pittsburgh Post-Gazette on Marcellus shale drilling.
Allegheny Front, environmental public radio for Western Pennsylvania.
The Environmental Defense Fund wants federal, not state rules, on hydraulic fracturing or fracking, Bloomberg News reports.
The eco-group says states that regulate the process to free up natural gas fail to meet international standards developed to prevent damage to natural resources.
"There are regulations in place in many states. They're not strong enough. I don't know of any state that has everything in place that's needed," said spokesman Eric Pooley.
The Paris-based International Energy Agency said the industry and policy makers should follow guidelines regarding chemical disclosure, well design and waste waste disposal to avoid opposition.
The industry says state regulations are very effective, said spokeswoman Amy Emmert of the American Petroleum Council.
About 26 states including Ohio have been reviewing and revising fracking legislation. Some are pushing for federal rules instead.
Fracking now accounts for 35 percent of U.S. natural gas production, up from 1 percent in 2005-2006.