Tests are being carried out in Lancashire by a firm exploring a new way of extracting natural gas from underground rocks.
A well is being drilled on land near Kirkham to determine the scale of shale gas reserves there.
Cuadrilla Resources believes Lancashire could potentially provide up to 10% of the UK's gas supply.
Extracting shale gas involves drilling deep into rocks and breaking them up with water and chemicals.
Pockets of gas are then released.
The gas being targeted by Cuadrilla is 8,000 to 10,000 ft (2,500 to 3,000m) below the surface, in rock which runs from Pendle Hill near Preston to the Irish Sea.
Although the tests are the first of their kind in the UK, shale gas is widely used in the United States, accounting for 45% of the country's gas supply.
Shale gas has previously been considered too expensive and problematic to retrieve, but new hydraulic fracturing technology has made the process more viable.
There are some environmental concerns, however, as it is thought the drilling could pollute ground water supplies.
Cuadrilla said it is working with the Health and Safety Executive and the Environment Agency to ensure there are no problems with the work it carries out this week.