The method apparently deposits an invisible chemical on the blank paper, which can then be revealed with the correct developing solution.

BBC: Spy tactics revealed by book

Fracking is a drilling process that frees gas from deep rock deposits by injecting wells with chemical-laced water at high pressure.

WSJ: Record: Libous ended stake in firm eyeing fracking

The bans stem from local residents' concerns about potential adverse impacts if the state lifts its 5-year-old moratorium on gas drilling using high-volume hydraulic fracturing, or fracking, which frees gas from deep shale deposits by injecting wells with chemical-laced water at high pressure.

WSJ: NY appeals court to consider local fracking bans

More than 10 drillers have already received Department of Environmental Protection permission to use mine discharges for fracking, a technique in which millions of gallons of water, along with chemical additives and sand, are pumped down a well to break apart gas-bearing shale deposits.

WSJ: Pa. pushes drillers to frack with coal mine water