A World War Two mine found in the Firth of Clyde contained about 350kg (771 lbs) of explosives, the Royal Navy has said.
The "pristine" German-laid mine was discovered by the crew of a Marine Scotland research boat near Wemyss Bay on Wednesday.
Seven crew members were evacuated by lifeboat before their vessel was sailed to Ettrick Bay on the Isle of Bute.
Bomb disposal experts then carried out a controlled explosion of the mine.
Lieutenant Commander Mark Shaw, Commanding Officer of of the Royal Navy's Northern Diving Group, said the submarine-laid mine was in "remarkable" condition considering it had been in the water for 80 years.
He added: "From the initial pictures, we were able to easily identify the mine type and importantly determine that the explosive fill was intact and therefore presented a significant hazard.
"This highlights the remaining presence of historic ordnance. Even small items can be unstable and present an explosive hazard.
"Carrying-out a controlled explosion is the only safe way of dealing with them and neutralising the hazard."
He said that anyone who comes across a suspected piece of ordnance should not interfere with it and immediately contact the emergency services.
The Scottish government said the marine research ship had quickly alerted the emergency services and other agencies once the mine had been discovered.
A spokesperson said: "The safety of our staff and crew remained of paramount importance as we worked with the Maritime and Coastguard Agency to coordinate the emergency response. At all times the incident was handled in order to minimise the risk to the public."