Completing a marathon on land is hard enough, but imagine doing that under water with kit weighing 40kg.
That is exactly what 14 Royal Navy divers are attempting to do, all in the name of charity.
The mine clearance divers from Fleet Diving Squadron at Horsea Island hope the punishing 26-mile challenge will raise £5,000 for the Royal Navy and Royal Marines Charity (RNRMC).
The divers begin the marathon tomorrow, with each pair taking it in turns to walk one kilometre in six metres of water in the giant lake at their base.
Their aim is to complete the challenge in 30 hours.
Their equipment includes lead-lined boots, an air cylinder and a 17kg helmet. They will pull a rigid inflatable boat which will carry three support crew and back-up equipment.
One of the divers, Leading Diver Duncan Watt, 37, said: ‘It will be extremely tough – both mentally and physically, especially the last 10 or so hours.
‘We haven’t had much time to do specific training for this because of our busy day jobs, but we are all confident that we can pull this off.’
Laura Young, the RNRMC’s fundraising executive, said: ‘This is by far one of the more eccentric events we’ve been involved with and we can’t wait to see how the guys get on.
‘Taking part in a marathon on dry land is hard enough as it is, let alone being underwater. I wish them every success.’
The charity has already shown its support for Royal Navy divers at Horsea Island.
This summer it supported a families day for the Defence Diving School and helped fund an outside recreation area and refurbishment of mess accommodation.
Donations to the challenge can be made at http://uk.virginmoneygiving.com/fleetdivingunit1.
The RNRMC work with families and veterans, especially those experiencing difficulty with child support, ill-health, old age or hard times.
For more info go to rnrmc.org.uk.