Jade Thomas, 23 and her mum Helen were walking across Mitchell's Fold and Corndon Hill, on the Powys/Shropshire border, with dalmatian Prince when they found what is believed to be a pre-World War Two device.
Miss Thomas spotted the device on a previous walk but forgot all about it before returning to the area with her mum last Saturday, May 31.
She said: "I was telling my mum about it and then without looking for it, we came across it.
"It was surprising because it was on common land and it is quite overgrown, with only sheep tracks to walk on.
"It was brown and quite camouflaged but not buried at all, so I was surprised that no one had come across it before."
The next day Miss Thomas contacted the police who arrived at her home within an hour to find the object before a controlled explosion was carried out.
"I didn't have a camera the first time I found it, " she said.
"I knew not to touch it, but didn't know what it was.
"I didn't think it would be a case of contacting the police at first and I wouldn't want to waste their time.
"But I e-mailed them and it clearly was important because they called back almost straight away."
Constable David Harte, of West Mercia Police, said: "I can confirm that on Sunday (May 31) at approximately 11.30am, West Mercia Police were alerted by a member of the public who had discovered an unexploded device within the marshland area of Corndon, White Grit.
"In view of public safety, the area was cordoned off to allow the Explosive Ordinance Disposal Unit to attended the scene and destroy the believed pre-World War Two device.
"Had it not been for a member of the public discovering the device, this could have potentially caused serious harm to the community.
"Should you find any suspicious devices, then we urge you to keep a safe distance and report them straight away by calling police on 101."
The object was about seven or eight inches long and was rusty, resembling artillery. Jade, a human resources officer, who lives in White Grit with boyfriend Ben Llewellyn, said she had done some research into what the device could be.
She said: "I think it is a mortar – one of my friends' boyfriends used to work in something to do with it and he said it looks like a mortar.
"I have tried to research it a bit more because I am interested in what it could be, but I would really like it if people who knew more about it came forward.
"I'd just really like to know when it got there and how long it had been there."
A mortar is an artillery weapon which fires explosive shells – known as mortar bombs.
They are fired at close range targets and have been used since medieval times. The indirect-fire weapon drops the bomb onto the target from above, rather than being aimed straight at it.
And the discovery it is not the first unexploded bomb to be found in the county, an unexploded World War Two device was found in a garden shed in Shropshire.
Richard Perry made the shock discovery when he was clearing out his late father-in-law's home in Childs Ercall, near Newport.