Ryan Junor was metal detecting with his teenage son and friends on Tuesday night in Invergordon, Ross and Cromarty, when they found the device that they originally though was a bottle.
An online post later revealed it is likely to be an unexploded First World War mortar bomb and, after police were alerted, they cordoned off roads in the town, shut schools and nurseries and called in bomb disposal experts.
Mr Junor told of his panic on realising he had carried the unexploded device several hundred metres.
The 39-year-old said: “My friend phoned me last night at around 5.30pm and asked if I wanted to go to the old bomb shelter area to do a bit of detecting to try the machine.
“I hadn’t used the machine before or been into that sort of stuff. My son loves the World War stuff, so I thought we might find a coin. The first thing that comes up when we use the machine is a bomb.
“I just started walking across the field and 10 minutes in, the noise started going on the detector. We stopped and dug around half a metre and found this large object, probably a bit bigger than a bottle of vodka. It was shaped like that as well.
“We thought it was just a bottle, or a old (spent) shell, but we just carried on detecting further down the hill. I took photos of it and put it on a metal detecting Facebook page and people came back on the page saying they think it is a Stokes mortar bomb from the First World War.
He added: “They said leave it where it is and call authorities.
“We kind of panicked then because we had it. I had carried it out a couple hundred yards into the field. We put it down straight away and called police and we had to wait for police and explained it to them.
“They told us the bomb squad was coming and to head home, which we all did, and I got up in the morning and all the local schools had been closed, and whole area closed off.
“We were pretty shocked when we realised that it had the potential to be a bomb, you know especially when we were digging around it and I was carrying it.
“I did not feel it was a bomb, or we would have gone nowhere near it. We would have just left it and called the authorities.
“It wasn’t till I was sitting in bed last night thinking, this is pretty scary. Everything is still closed now, all cornered off. My kids are off school today as local schools are closed.”
Mr Junor said he had only had his metal detector for three days before making the discovery.
Roads around the area in the Highland town have been cordoned off following the discovery and police are urging people to stay away from the area.
Invergordon Academy, Park Primary School and nursery, and South Lodge Primary and nursery are closed while emergency services deal with the incident.
A Royal Navy spokeswoman said: “We can confirm that Charlie Squadron from the Faslane-based Diving Threat and Exploitation Group (DXTG) are attending the incident in Invergordon.
“They have dispatched a Royal Navy Bomb Disposal Officer to the scene and are liaising with Police Scotland to deal with an item of historic ordnance which was found by a metal detectorist.”
Part of the road between the junctions of Gordon Terrace and Academy Road is shut, and Davidson Drive is closed to vehicles and pedestrians.
A Police Scotland spokeswoman said: “Emergency services are in attendance at Castle Terrace, Invergordon, after an unexploded ordnance was found.
“A cordon has been put in place and explosive ordnance disposal (EOD) has been contacted.”
Invergordon is a deep water port which served as a naval base in both world wars.