A hand grenade blown up in a controlled explosion in Hellesdon yesterday had been fished out of the river by a man who was 'magnet fishing'.

Eastern Daily Press: Marlpit Community Garden. In the foreground is a small hole, which may be where the army bomb squad detonated the hand grenade found in the River Wensum on Sunday. Picture: Stuart AndersonMarlpit Community Garden. In the foreground is a small hole, which may be where the army bomb squad detonated the hand grenade found in the River Wensum on Sunday. Picture: Stuart Anderson (Image: Archant)

Police said the man who found the grenade, which may date from the Second World War, first took it home before calling the authorities.

Nearby residents said their houses shook when the army's bomb squad exploded the device near the Hellesdon Road bridge on Sunday evening.

Magnet fishing is the equivalent of beachcombing, and involves uses a strong magnet attached to the end of a rope to try and grab ferrous metal objects that are hidden or lost underwater.

A Norfolk Constabulary spokesman said: 'We were called at 2.20pm to reports of a hand grenade that was found in the River Wensum.

Eastern Daily Press: Marlpit Community Garden, where the army bomb squad detonated the hand grenade found in the River Wensum on Sunday. Picture: Stuart AndersonMarlpit Community Garden, where the army bomb squad detonated the hand grenade found in the River Wensum on Sunday. Picture: Stuart Anderson (Image: Archant)

'The person who found it actually took it home with them.

'The informant was fishing at the bridge in Marl Pit Lane (Hellesdon Road). He was magnet fishing, and that has stuck to the grenade.'

The spokesman said the grenade was found about midday and the bomb disposal unit was on the scene at 6.44pm.

MORE: Grenade blown up in Norwich in controlled explosion

Eastern Daily Press: Marlpit Community Garden, where the army bomb squad detonated the hand grenade found in the River Wensum on Sunday. Picture: Stuart AndersonMarlpit Community Garden, where the army bomb squad detonated the hand grenade found in the River Wensum on Sunday. Picture: Stuart Anderson (Image: Archant)

Police put a cordon in place and the spokesman said the grenade was destroyed in a controlled explosion at the Marlpit Community Garden, which is next to the river, just after 7.15pm.

But today there was little sign of where the blast took place, other than a small hole which Jim Froud, a garden trustee, said was not there before.

Mr Froud said he got a call to ask permission to blow up the grenade on the site, which has a large open field not used to grow crops. He said: 'When you have a controlled explosion you have to find a location out of the range of trees and power lines. They will have carried it in a box and put in a hole in the ground.'

Bryan Cameron, who lives nearby, said he was at home watching TV when he heard the explosion.

He said: 'It was a good old bang. We came out to have a look and we saw the white bomb squad van and three police cars park.Then after the bang they just got back in their cars and drove away.'

Joni Dorian-Wright, 23, who lives in Hellesdon Road, was also among those who heard the blast. She said: 'It shook the house quite a bit and we quite distinctly felt it.'

Police said the cordon was lifted just before 8pm, and no road closures were necessary.

What is magnet fishing?

Magnet fishing is a growing craze among traditional anglers around Britain's lakes, shores, canals and rivers.

And some amazing finds have been made over the past couple of years. Among items brought up from British waters include a Second World War Enigma machine, pistols and even a Civil War cannonball.

And thousands more pedestrian items such as bottle caps and keys have also been retrieved from the muddy depths.

'Magnateers', as they are known, use super strong magnets attached to hardy ropes, which are then flung into the water in the hopes of they will stick to something interesting - or valuable.

Pure gold cannot be detected by these magnets, however most coins contain metal that can.

Most practitioners try to observe landowners and local authority regulations concerning outdoor waters, and will seek permissions where appropriate.