Is the controversial extraction of shale gas by fracking safe?
Never heard of "fracking"? If not, chances are you will soon. It is short for hydraulic fracturing, and is part of a process by which the United States is tapping into a vast new source of energy - natural gas trapped in shale rock, deep underground.
But this new source of energy is controversial. Video sharing website YouTube is buzzing with clips showing people who live close to gas drill sites setting light to their tap water.
They claim this happened only after drilling released methane gas and contaminated their private water wells.
There is a lot at stake - not just money, but also the reputation of a whole new industry.
Some estimates suggest there is enough shale gas under US soil that in energy terms it represents at least a couple of Saudi Arabias. What is more, this trillions-of-dollars-worth of energy is home-grown, and cleaner than other fossil fuels.
Until recently it was thought too difficult to tap economically. But a new engineering approach that combines "fracking" with horizontal drilling has challenged that (see how fracking works in the video below).
If all goes well in the US, Europe could be next. Just this September, a Chatham House report weighed up the prospects of a shale gas revolution.
But is it safe to go ahead?
'Learning by doing'
In early October, I went to the town of Dimock in the US state of Pennsylvanian to find out more. Residents there have become well known for their experience with fracking, and with the gas companies at work in their backyards.
The message I took away from the trip was similar to that highlighted by Chatham House, which in its report spoke about the industry as one which is "learning by doing".
When I spoke to one of the gas companies operating in Pennsylvania, Chesapeake Energy, I found that such "learning by doing" had uncovered a problem.
The company, the second largest gas company in the US, conceded this straight away. Brian Grove, from Chesapeake, told us that problems the company had encountered with shallow pockets of gas could explain how methane might reach people's drinking water.
The threat of methane in people's drinking water is one of two chief safety concerns about the industry.
If colourless, odourless methane gas migrates into people's private drinking water wells it is not a health risk in itself, though in high concentrations methane gas is an asphyxiate.
More worrying, the gas could explode if it collects in a confined space.
The second anxiety is over what is in the so-called fracking fluids. These are mixed in with millions of gallons of water, and pumped underground at high pressure to help ease the gas out of the dense shale rock.
One of the Dimock residents we met, Bill Ely, like many landowners in the area, leased his land to a company called Cabot Oil and Gas, hoping to make money from royalties.
Now he is suing the firm for contaminating his water supply with methane gas and putting his home at risk of explosion.
After a neighbour's private water well apparently did explode, Cabot installed ventilation pipes on Mr Ely's water well, and agreed to divert his well water through a hose, rather than into his home. They truck in all his drinking water too.
(You can see Mr Ely setting light to gas coming off his water in the video below)
George Stark is the spokesman for Texas-based Cabot, which has already invested $900m in exploiting gas in just this one county in Pennsylvania.
He says people have long been able to light up their water round in the area because of naturally-occurring methane:
"Our experts have checked over our casing, cementing, the drilling practice itself, the tubular that we're using to go down and they have determined that at this point that there's no Cabot operations that's occurring allowing for the discharge of methane into the waters," he told me.
But the state's environmental regulator says he has the equivalent of fingerprints linking the methane in local wells and Cabot's operations.
So there's a standoff.
Vast tracts of North America sit on top of the ancient shale rock that holds natural gas in tightly compressed layers. Extraction or exploration is now underway in 30 US states, with attention focussed on the so-called Marcellus shale under Pennsylvania and New York states.
And in New York, they are watching Pennsylvania with interest. The state has a moratorium in place while its regulators weigh up the pros and cons.
There is growing interest in shale gas in the UK too. This summer, Cuadrilla Resources, a UK company, began test drilling near Kirkham in Lancashire.
Geoff Maitland, professor of energy engineering at London's Imperial College, told me there is probably significant potential in the UK, as yet unexplored:
"There are good indications both in the Lancashire area, and in Dorset in the onshore Kimmeridge shales. Scotland also has good prospects," he said.
Tony Ingraffea, is a civil engineer from Cornell University in the US, with expertise in fracture mechanics. He has spoken out against the way shale gas drilling is being carried out:
"You have steel casing, surrounded by cement, surrounded by rock. If any of those protection barriers fails we have an open pathway. A faulty cement job can be a failure by which gas or other fluids can find their way to the surface."
And it seems Chesapeake Energy might have hit upon at least one explanation for people's flaming tap water. The company told us that it had been forced to change its drilling mixture earlier this year, adding more latex.
Why? Because, just as Mr Ingraffea feared, problems with cement had allowed gas to migrate outside the well casing.
Brian Grove, from Chesapeake, explained: "In some cases it looks like, as the cement was drying, high pressure shallow methane kept it from drying properly, and would allow channelling to develop on the outside of the casing, which then could allow methane to move upwards through the shallower zone and to get into fresh water."
Mr Ingraffea is not surprised, but does not think this is the right approach: "I don't accept the notion that that industry can come in and say 'we're safe - oops, wait a minute, we found another mistake, we found another situation we hadn't anticipated, we're learning while we're doing'."
Much of the suspicion about the natural gas industry dates back to 2005, when US President George W Bush signed an energy bill which granted it exemptions from federal regulations, including the Safe Drinking Water Act.
Then-US vice-president Dick Cheney, the former CEO of Halliburton, worked prominently on energy policy at the time.
Halliburton is one of the major makers of fracking fluids, but appears to be the most reluctant to disclose their chemical make-up.
Earlier this month, the company was issued with a subpoena by the US Environmental Protection Agency (EPA) to force it to disclose more. Halliburton said it has worked to supply the EPA with the information it wants.
And as the gas companies expand their work across the US, this is a debate that is going global. As we finished filming in October, China's state gas company announced plans to invest in Chesapeake Energy's oil and gas fields.
The growing interest in the industry is not all positive. A documentary called Gasland has fired up the debate.
While New York State weighs up its position, many landowners welcome the industry, and the jobs and wealth it brings. Others say they are determined to secure the best deal for themselves, and for the environment, from this new gas Gold Rush.