An oil company has started drilling for gas off the Pembrokeshire coast.
The rig is expected to be in place until mid-November
A rig and two support vessels manned by around 120 workers are in place about 21 miles west of St David's Head.
Marathon Oil hopes to know by November whether there is enough natural gas under the Irish Sea to make exploration commercially viable.
The company has looked for gas under the bed before but the LNG plans for Milford Haven has renewed the company's interest in the area.
Marathon Oil, which has interests in the North Sea, Norway and Kinsale Oil off Ireland, first explored the Dragon Field, on the median line between Wales and Ireland in 1994.
Company project manager Mike O'Neill said: "The rig is out on location now and is drilling.
"Everything is moving forward on schedule. We are supporting the rig from Pembroke Dock and we have set up a logistics office.
"There's about 80 people on the rig itself and two support vessels with about 20 on each vessel."
Drilling is due to end by mid November depending on the weather.
"Basically if it's bad news we will know sooner rather than later - maybe by the middle of October.
"If it's good news it will take us quite a while to evaluate it and we probably will not have a full evaluation before March.
"The whole project swings on the volume of recoverable reserve from the field. That decides whether the project is economically viable given what we would have to spend to recover it."
The gas exploration follows plans by Malaysian company Petronas to ship LNG - gas cooled down to liquid form to make it easier to transport - to two new terminals planned for Milford Haven.
It would turn Milford into a major centre for LNG from all over the world and make it a major supplier to the UK gas supply.