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This story and infographic were updated on April 28, 2016.

As debate intensifies over oil and gas drilling, most states with frackable reserves are already fracking—or making moves to do so in the near future.

That translates to 21 states, from California to Texas, Michigan to West Virginia, currently employing this high-intensity form of energy extraction, and five others may soon follow. Called high-volume hydraulic fracturing, or fracking, the controversial process became commercially viable in the late 1990s. It generally involves injecting millions of gallons of water, along with sand and chemicals, down a well to extract oil-and-gas reserves that were previously hard to access.

InsideClimate News compiled a comprehensive map of the nation’s fracking activity. This state-by-state breakdown will be periodically updated.

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The breakdown

Fracking is used differently in each state, depending on the available fossil fuels. Texas has thousands of wells that tap into deeply buried shale deposits. By contrast, in Indiana, fracking occurs for a small percentage of wells.

Tennessee and Kentucky are outliers. While both states allow high-volume fracking (modern fracking), drillers there tend to use other extraction techniques that can involve injecting nitrogen gas underground.

Illinois and North Carolina are the most recent states to allow modern fracking, with their state legislatures passing new rules in 2015 and 2014, respectively, and regulators are now waiting for applications. Nevada allows the process and had active operations as recently as December 2014; regulators there are not sure why operations have since stopped. Meanwhile, drillers in Alaska are exploring fracking’s potential. But there’s been little interest in Florida, which technically allows the practice.

New York and Maryland buck the trend. New York was the first  state with sizeable fossil fuel reserves to close its borders to fracking. In December 2014, Gov. Andrew Cuomo banned the practice. His decision cited the public health risks from water and air pollution, and the unknown climate change impacts of extracting natural gas. Maryland lawmakers, also concerned about fracking’s impact on the environment and public health, passed a moratorium in May 2015 that bars the process until October 2017. The extent of the natural gas resources underneath Maryland isn’t known, according to state geologists.

Two other states—Massachusetts and Vermont—also prohibit fracking. These drilling restrictions are primarily symbolic because neither region has major resources.

Of the remaining states, some, including Rhode Island, have no known deposits of oil or gas. Georgia and other states have small reserves that could be developed at some point if producers determine it’s economically feasible.

Despite many state regulators’ embrace of fracking, there’s growing local resistance. In California, the nation’s third top producer of oil, a combination of counties and towns have voted to ban fracking within their boundaries.  Hundreds of towns, tribal territories, cities, and counties across the country have created similar bans or moratoria in recent years.

Lawmakers in some states have retaliated. Energy regulators in Texas, Oklahoma and North Carolina passed restrictions last year on local communities’ ability to limit fracking and related activities. State officials and energy companies have also sued communities over local fracking-related restrictions with varying success. For example, the energy industry last year succeeded in blocking local bans in Ohio’s Broadview Heights and Munroe Falls.