The 30 divers will spend the next few days surveying the remains of seven German ships that still lie on the seabed in Scapa Flow 100 years after they were scuttled following the First World War.
They aim to identify and retrieve nets, ropes, creels and other man-made debris littering the renowned wreck site, where it poses a danger to marine wildlife and the environment.
The initiative is the fourth annual clean-up carried out by volunteers from the environmental group Ghost Fishing UK.
Discarded, lost and abandoned gear takes hundreds of years to break down and continues trapping sea life long after its commercial use is over.
The United Nations estimates that 640,000 tonnes of fishing nets are dumped in oceans around the globe every year, killing and maiming marine life.
It could take as long as 600 years for these nets to break down in the water – gradually fragmenting into smaller and smaller pieces, eventually becoming microplastics.
Christine Grosart, secretary of Ghost Fishing UK, is with the team in Orkney, where the first dives have already taken place.
“We’re retrieving old and lost fishing gear,” she said.
“We’re not taking out active gear, we leave that alone.
“We’re going for lost nets and pots that continue to catch and kill and also act as bait, encouraging more animals to come in and get killed in a horrible cycle where nobody wins.
“We’re trying to remove all that, and of course as a side product we’ve got the ocean plastics as well, which break down and get quite nasty.”
The divers are working alongside scientists from Heriot-Watt University and the anti-cruelty organisation World Animal Protection on the Scapa Flow project.
As well as coordinating the clean-up, Ghost Fishing UK will also be providing specialist training to divers from across the UK.
“There are a few specific skills divers need to be able to do this kind of work, which can be quite dangerous,” said Ms Grosart.
“Dealing with fishing nets underwater and trying to get them to the surface, there’s a lot that can go wrong.”
She added: “Scapa Flow is a world-famous diving destination. Most of the ships were salvaged but there are three battleships, three light cruisers and a fast mine-layer still down there.
“They don’t count as war graves so we can dive them without any issues. Because the wrecks are so big, fishing gear does get snarled and snagged on them. Fishing boats trawl very close to the wrecks, but the closer they get the higher the risk of losing gear.”
The team hope the latest clean-up will see the site in pristine condition for the centenary of the sinking in 2019.
An evening of free talks is being held in Stromness on Tuesday evening to provide information on the work being carried out, with presentations from Ghost Fishing UK, World Animal Protection and Heriot-Watt.