What are the mysterious booms shaking one Wisconsin city in the dead of night?

PUBLISHED: 06:03 GMT, 21 March 2012 | UPDATED: 06:03 GMT, 21 March 2012

A series of mysterious underground booms have been rattling a small Wisconsin town for at least two nights, baffling residents and local officials who were left stumped after ruling out numerous theories, from earthquakes to water pressure problems.

The noises - described as rumbles of thunder, sonic booms or fireworks - were being reported in northeast Clintonville, about 140 miles from Milwaukee, on Sunday night but quieten down at daybreak.

The sounds started back up on Monday night across a wider area and continued until 5am on Tuesday, said City Administrator Lisa Kuss.

'There's no warning, it's just ''bam,'" Ms Kuss said. 'I would describe it as startling, an adrenalin rush. Your heart is instantaneously kind of racing because you are not expecting it.'

Resident Al Miller said he has been hearing rumblings for a couple weeks but believed it to be thunder before an especially loud boom woke him around 3am on Monday.

'My house shook and it was just like a shock,' the 71-year-old said. 'I got out of bed and was like ''Wow.'' I thought one of my trees fell onto the house.'