Around 100 local people have met in Llanelli to discuss plans by an energy company to extract coal gas from deep under the Loughor Estuary.
Cluff Natural Resources wants to extract the gas using a process known as gasification.
It involves drilling a deep hole and igniting the coal before piping the gas to the surface.
The firm has been awarded a licence for the Loughor and Dee estuaries by the Coal Authority.
But it will need various permits before test drilling can start.
The Loughor Estuary project area in Carmarthenshire covers 42 sq km; the Dee Estuary project on the north Wales coast involves a 69 sq km area.
The public meeting was called by campaigners against the proposals, worried about the technology itself - and the potential impact on the estuary and local beauty spots.
Keith Ross, of Safe Energy, said: "My major concern is for the environment but we also have a more specific concern about this particular process and that it could lead to subsidence in the area."
Malcolm Ridge, chairman of the Gower Society, said it was a new technology which needed proper examination.
"We need hard evidence as to what the effects will be," he said.
The company has denied Deep Underground Coal Gasification (or UGC) is a threat to the environment or any risk to local people.
It says its UGC process is not the same as fracking - short for "hydraulic fracturing" - which has caused controversy.
That involves drilling deep underground and releasing a high-pressure mix of water, sand and chemicals to crack rocks and release gas stored inside.
The basic principle behind UCG is that coal seams are found up to 500m (1,641ft) underground - far too deep to mine.
Oxygen combusts with the coal-producing synthesis gas - a combination of carbon dioxide, carbon monoxide, methane and hydrogen.
The gas, or syngas, could then be piped to the surface via another borehole.
Both methods are part of the drive to find cleaner energy sources for the UK's future electricity needs, rather than burning coal, and cheaper alternatives than importing gas from abroad.
When the Loughor and Dee projects were initially announced earlier this year, Algy Cluff, chairman and chief executive of Cluff Natural Resources, called it a "low cost entry into underground coal gasification which is becoming a burgeoning industry".
He added: "It is well known that coal seams extend into the offshore waters around the UK and with proven technology now available to utilise this energy source, we intend to embark on the process by extracting gas from these coal seams.
"UCG has the potential to do much to address the UK's future energy needs, avoids the use of fracking and enables the gas generated to be easily controlled by the supply of oxygen."
Last year, another company said it wanted to drill for coal and extract the gas from under Swansea Bay.
Clean Coal Ltd has five licences around the coast of Britain and was trying to locate reserves which were off shore and too deep to be mined traditionally.
The Swansea licence ran out last November.
It is thought up to 1bn tonnes of coal could lie beneath the surface.