An earth tremor shook the south west of England on Thursday evening for about eight seconds, police in Somerset said.
A police spokesman said they got between 10 and 20 calls reporting what they called "movements" around 8.30pm.
He said: "We have no idea what's caused it, but it's being put down as an earth tremor." No accidents, damage or injuries have been reported.
A fireman described the quake: "The floor moved, and there was a deep rumble like a loud roll of thunder."