A team of Royal Navy divers have set themselves their toughest charity target to date - swimming all the way around Portsea Island.
It's thought to be the first time anyone has undertaken the gruelling 12-and-a-half mile swim, and they will have to cope with strong tides, thick mud and unpredictable currents.
Starting and finishing at Gunwharf Quays, the divers are part of Project Vernon - a campaign group trying to put a monument to navy divers in the shopping centre.
They will also donate money to the Royal Navy & Royal Marines Charity (RNRMC).
Last year the team swam the 10-and-a-half-mile length of Windermere in the Lake District to raise cash for the armed forces' charity, Help for Heroes.
But Lieutenant Commander Mick Beale said that swim was nothing compared to their latest challenge.
'It is an extreme swim, there's no doubt about it,' he said. 'The main problem is you have to predict the tides and use them in your favour.
'If we judge it wrong there is no way we'll get out of Langstone Harbour.
'Windermere was a straightforward swim. This will be far more tricky. I am confident it's possible though.
Lt Cdr Beale completed the swim as part of a relay team in the 80s, but says this might be the first time anyone has tried to swim a complete circuit of Portsea Island.
The planned monument - a sculpture of a navy diver defusing a mine - will be placed in Gunwharf Quays which, until 1996 when it stopped being HMS Vernon, was the Royal Navy's Diving and Mine Warfare School.
Fundraisers need at least 250,000 to buy the sculpture and have it installed in the water opposite The Old Customs House pub.
The Project Vernon team has so far raised more than 100,000.
Lt Cdr Beale said: 'It is important to remind people what this area used to be, and to recognise all the divers that have served and worked from here over the years.
'People don't tend to know much about the work they do, and a great statue like this will hopefully go some way towards changing that.'
The challenge this July will take between eight and 10 hours to complete. Go to vernon-monument.org