People living on Anglesey have been left baffled about the cause of a loud noise which shook their homes and made pots and pans rattle.
Anona Jones said she was in the kitchen of her home at Llanddaniel near Gaerwen when she heard a loud "rumbling" noise at 1425 GMT.
A spokesman at the British Geological Survey in Edinburgh said no seismic activity had been measured in the area.
But he added that a number of calls had been received about the noise.
"It was a very obvious noise, and I thought there had been an earthquake," said Mrs Jones.
"I was in the house with my husband and a local farmer, and we'd just sat down to have a cup of tea at about 2.25 this afternoon."
"We all heard this rumbling getting nearer and nearer the house.
"The cooker hood began to shake and the saucepans on top of the microwave rattled," she added.
Mrs Jones said she had rung her sister-in-law in Llanfairpwll and she too had heard the noise.
A spokesman at the British Geological Survey in Edinburgh said no instruments in Wales had measured a seismic event on Tuesday.
"We have had quite a few people from the Gaerwen area of Anglesey ringing us asking about it, but there are no reports of tremors, no detection that it was a seismic event," he said.
The most probable explanation was that it was some kind of sonic boom, which could be created by aircraft, he added.
"I don't know if that is what it was, but sonic booms have all the characteristics of this kind of event," he said.
In May 2006 people living on the island, as well as on the mainland of north Wales, were woken in the middle of the night by a tremor.
The shock wave measured 1.4 on the Richter scale at 0322 BST.