LONDON -- The waters around Norway's capital are considerably safer after a two-week hunt by the crew of HMS Cattistock.

Echoes of WW2 as HMS Cattistock clears mines around OsloExperts from the Portsmouth-based minehunter found more than ten WW2 mines and torpedoes during a concerted search for unexploded wartime bombs with a NATO task group.

Norway was occupied by the Nazis between April 1940 and the war's end, with the RAF making repeated attempts to disrupt shipping between Oslo and Germany - an operation codenamed ‘gardening vegetables,' with Oslofjord itself also given the cover name 'onions.'

Twin-engine Hampden bombers, or four-engine Lancasters and Halifaxes dropped between one and two dozen mines at a time, just one or two seconds apart from a mere 600ft over the fjord.

Around 1,500 British 'A' mines were sown in the fjord and its many inlets - some triggered by a ship's magnetic field, others by the noise vessels made as they passed overhead.

While the major sea lanes were cleared in the immediate aftermath of the war, the fjord covers 766 square miles (1,984 square km) with countless lesser inlets, estuaries and the like.

In fact, around 100 mines have been discovered by ships and divers an neutralise, but there remains a lot of unexploded ordnance in the fjord.

The latest sweep for 'vegetables' in the 'onions patch' was carried out by NATO's Minehunter Group 1 - Cattistock, plus two Norwegian and one German hunter.

"The environmental conditions were incredibly challenging - water temperatures down to freezing, often causing the formation of ice, and air temperatures consistently well below freezing. So, diving operations were unusually demanding," said Leading Diver Karl Atkin.

"We often went down to 60 meters in zero visibility and had to identify the mines by touch."

"The deep dives bring a heavy decompression penalty too - we had to conduct 'stops' at various depths on the way up. When we do, we're motionless in the water column and get very cold very quickly!"

The cold also forced the ship to abandon the last two miles of hunting due to thick sea-ice closing the bay.

"There were people walking their dogs on the frozen sea, where we should have been mine hunting!" said Cattistock's Commanding Officer Lt Cdr Charlie Wheen.

In all 27 mines and four torpedoes were found by the NATO group with the Brits accounting for more than one third of the haul.

Upon leaving Oslo, Cattistock passed over the wreck of the German heavy cruiser Blucher, sunk in narrows about 16 miles south of the city during the opening minutes of the German invasion.

The brand-new ship - crammed with troops as well as her crew - was torpedoed as she passed historic Oscarsborg fortress and capsized minutes later, killing upwards of 800 soldiers and sailors.

Passing over the wreck - a protected war grave - Cattistock used her Sonar 2193, which can locate a football at depths up to one kilometer, to scan the 200m-long hull of Blücher.

"Because of the orientation of the wreck, the sonar scan only shows the underside of the keel in a long thin strip," said Lt Cdr Wheen. "Sonar 2193 doesn't provide the same fidelity as the survey ships, but it was still exciting to see it on the screen."

"Overall it's been an excellent period of tasking in a beautiful part of the world - but in very challenging conditions."

Source: Royal Navy