It takes brute force to wrest natural gas from the earth. Millions of gallons of chemical-laden water mixed with sand -- under enough pressure to peel paint from a car -- are pumped into the ground, pulverizing a layer of rock that holds billions of small bubbles of gas.

The chemicals transform the fluid into a frictionless mass that works its way deep into the earth, prying open tiny cracks that can extend thousands of feet. The particles of sand or silicon wedge inside those cracks, holding the earth open just enough to allow the gas to slip by.

Gas drilling is often portrayed as the ultimate win-win in an era of hard choices: a new, 100-year supply of cleaner-burning fuel, a risk-free solution to the nation's dependence on foreign energy. In the next 10 years, the United States will use the fracturing technology to drill hundreds of thousands of new wells astride cities, rivers and watersheds. Cash-strapped state governments are pining for the revenue and the much-needed jobs that drilling is expected to bring to poor, rural areas.

Drilling companies assert that the destructive forces unleashed by the fracturing process, including the sometimes toxic chemicals that keep the liquid flowing, remain safely sealed as much as a mile or more beneath the earth, far below drinking water sources and the rest of the natural environment.

More than a year of investigation by ProPublica, however, shows that the issues are far less settled than the industry contends, and that hidden environmental costs could cut deeply into the anticipated benefits.

The technique used to extract the gas, known as hydraulic fracturing, has not received the same scientific scrutiny as the processes used for many other energy sources.

For example, it remains unclear how far the tiny fissures that radiate through the bedrock from hydraulic fracturing might reach, or whether they can connect underground passageways or open cracks into groundwater aquifers that could allow the chemical solution to escape into drinking water. It is not certain that the chemicals - some, such as benzene, that are known to cause cancer - are adequately contained by either the well structure beneath the earth or by the people, pipelines and trucks that handle it on the surface. And it is unclear how the voluminous waste the process creates can be disposed of safely.

"This is a field where there is almost no research," said Geoffrey Thyne, a former professor at the Colorado School of Mines and an environmental engineering consultant for local government officials in Colorado. "It is very much an emerging problem."

The lack of scientific certainty about hydraulic fracturing can be traced in part to the drilling industry's success in persuading Congress to leave regulation of the process to the states, which often lack manpower and funding to do complex studies of underground geology. As a consequence, regulations vary wildly across the country and many basic questions remain unanswered.