A 600lb Second World War mine was detonated in Swanage Bay creating a massive plume of water and shock waves which shook the town.

The huge device, which still contained highly explosive material, was exploded by Navy bomb disposal experts at 12.30pm on Wednesday.

It sent a 30ft plume of water into the air and shook the floor beneath the feet of unsuspecting Swanage residents.

Advertising executive Sarah Crabb said: “We didn’t hear it, but we felt the ground shake for a good couple of seconds – which is a long time when that happens.

“We didn’t know what it was – the whole ground shook.”

The device was discovered by divers on Tuesday, who photographed it an alerted the Coastguard.

They notified bomb disposal experts, who used the image to identify the bomb as a Second World War ground mine which would have been dropped by an aircraft. It had lain undiscovered from more than 60 years.

Coastguard watch officer John Braisher said the bomb disposal team hired Swanage dive boat Skua and located the mine a kilometre east of Ballard Point.

An exclusion zone was put in place and Coastguard teams from Swanage and St Aldhelm’s cleared the public from Ballard Point. Police RIBs were deployed to maintain the zone at sea. The device was then detonated in a controlled explosion.

Mr Braisher said: “The big bang went off at around 12.30pm. Water went about 30ft into the air. We received three 999 calls about it.”