The 300 competitors, many wearing fancy dress, ran, crawled and scrabbled across the thick black mud of the River Blackwater in Essex at low tide and back yesterday.
Each year competitors attempt to dash across the bed of the river near Maldon, and luckily this year the sun was out to keep the mud-racers warm.
The crazy competitors crawl through metres of sticky mud to reach the finish line before the tide comes in. The event sees a number of competitors dressed in wacky costumes, which makes the task of crossing the finish line somewhat harder.
Sticky: Participants reach the finish line of the annual Maldon Mud Race in Maldon, Essex
It mud be love: A couple dressed as a bride and groom racing around a course on the mudbanks of the river Blackwater at low tide
Competitors race around a course on the mudbanks of the Blackwater river at low tide. The first edition of the race was held in 1973
In the thick of it: A competitor in distress attempts to crawl through the muddy sludge
In trouble: A competitor tumbles down the mudbanks of the Blackwater river
Meanwhile, an even bigger event in Northamptonshire saw thousands of people taking part in an 11 mile endurance event complete with 22 military-style obstacles.
Around 14,000 participants would have been left uttering 'muddy hell' as they descended on Boughton House in Kettering as they aimed to complete the gruelling Tough Mudder circuit.
Big names also taking part in the mud run were England and British and Irish Lions assistant coach Andy Farrell and attacking skills coach Mike Catt, who both completed what organisers describe as 'probably the toughest event on the planet'.
At the finish line there was no prize for 'mudders' - who fought through fire, plunged into icy water, scaled 12ft walls, waded through muddy swamps, crawled through underground tunnels and navigated wires electrically charged with 10,000 volts - other than a Day-Glo orange headband and a T-shirt to mark their success.
The name of the game is teamwork and camaraderie because it is almost impossible to finish the course alone, a Tough Mudder spokeswoman said.
'The big thing is that it's not a competition, it's not a race. It's a challenge so a lot of the obstacles you have to do in a team,' she said. 'You get headbands at the end to show you've completed the course. It's about teamwork, camaraderie, bonding.'
Muddy waters: Competitors leave the freezing water and enter the thick mud
Out the otherside: The competitors cross the river then get reduced to a crawl
Each year competitors attempt to dash across the bed of the river near Maldon, and luckily this year the sun was out to keep the mud-racers warm
The event sees a number of competitors dressed in wacky costumes, which makes the task of crossing the finish line somewhat harder
Charity fund raiser Joel Hicks celebrates as he scrambles up the river bank to the finish line of the annual Maldon Mud race in Maldon, Essex
A participant recovers after taking part in the annual Maldon Mud Race in Maldon, Essex
Competitors shower after Competitors shower after scrambling though 400-yards of foul-smelling muck for the annual Maldon Mud Race
Mudders taking part yesterday were treated to a bit of extra dirt and dampness as rain and hail fell on the Northamptonshire course, but today's participants benefited from some rare bank holiday weekend sunshine.
As the event has grown in popularity it has seen more of a mix of people taking part and more women are now taking on the challenge, and more mixed sex teams are entering, the spokeswoman said.
Sarah Harvey, Tough Mudder UK's vice president, said: 'After an overwhelming reception last year, 2013 is set to be a really exciting year for Tough Mudder and we definitely started with a bang in Kettering.
The rain and the hail added to the tonnes of mud on site as the course lived up to its name. Participants braved muddy trenches, and waist-high swamps
The course was designed by British Special Forces. On average, only 78 per cent of participants finish the course, while average time is 2.5 hours
The course features a 12-15 mile trail run, containing more than 20 military-style obstacles
England and British and Irish Lions assistant coach Andy Farrell take part in Tough Mudder 2013 at Boughton House, Kettering
England rugby attack coach Mike Catt takes part in the challenge today
While the south of the country will continue to bask in the blazing heat tomorrow, it is likely to remain cloudy with outbursts of rain in the north east and Scotland
Obstacles differ from place to place, with each and every one presenting a challenge of the body and mind
'The London North Mudders were amazing and not only braved one of our toughest courses yet, but also the erratic weather!
'We're now looking forward to Matterley Bowl in June for what should be another a great event.'
Founded and run by Englishmen Will Dean and Guy Livingstone, Tough Mudder has already established itself in America, Australia and now the UK since its launch in May 2010, and Tough Mudder recently signed up its one millionth participant worldwide.
Tough Mudder has increased the number of UK events from three to six this year with the next event London South taking place at Matterley Bowl near Winchester on June 8 and 9.