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A controlled explosion was carried out on a live hand-grenade.
Cameron Carron, from Gillingham, stumbled across the explosive while he was fossil hunting at Warden Point on Sheppey.
The 18-year-old alerted the coastguard to the discovery at around 4.15pm yesterday and part of the beach was cordoned off.
An explosive ordnance disposal (EOD) team was called to the site and confirmed the device was live.
A controlled explosion was carried out at around 8.30pm.
Cameron said: "I was very shocked when I realise it was a hand grenade. I first thought it was just a piece of old rounded metal.
"I've now learnt to be careful what you pick up whilst fossil hunting and to always double check what I collect.
"I was just so surprised to see something like that as it was such a dangerous find.
"When the EOD told me that it was live I suddenly felt very lucky to be alive, as it could of exploded at any time.
"It was a shame I couldn't see the grenade being exploded because of the time and safety.
"I just wish it wasn't live so I could have kept it."
A Maritime and Coastguard Agency spokesman said: "Yesterday we received a 999 call from a member of the public reporting suspected ordnance.
"Sheppey Coastguard rescue team were sent to establish a cordon around the ordnance and an EOD team were contacted to coordinate disposal.
"Police were made aware and a controlled detonation was carried out by EOD yesterday."
Police and the Ministry of Defence have been contacted for more information.
Military hardware washing up on Sheppey's beaches is quite common as the Thames Estuary was used for dumping unwanted items back to the time after the Crimean War.