More than 10,000 adrenaline junkies took on 'America 's toughest obstacle course ' in California on Sunday with a photographer on hand to document their pain-stricken faces . Lucy Nicholson of Reuters snapped away at various points on the 12-mile Tough Mudder course in San Bernardino as contestants courageously made their way through mud pits , iceberg baths and fields of live wires in almost 90-degree heat . One shot shows a man in seeming agony as he 's shocked by electric wires while another shows a contestant welling up as he crawls through a haze of tear gas . Later in the course , a couple were photographed sharing a passionate embrace after navigating a tunnel of barbed wire . Both of their T-shirts appear coated with dirt . Scroll down for video . Hair-raising : More than 10,000 adrenaline junkies took on 'America 's toughest obstacle course ' in California on Sunday with a photographer on hand to document their pain-stricken faces - here a man screams as he is shocked by electric wires . Plop : Spectators watch on as Tough Mudder participants wriggle through pipes and plunge into a pool below . Crossing the line : Lucy Nicholson of Reuters snapped away at various points on the 12-mile Tough Mudder course in San Bernardino as contestants courageously made their way through mud pits , iceberg baths and fields of live wires in almost 90-degree heat . Caught on camera : One shot shows a man in seeming agony as he 's shocked by electric wires . Tough Mudder , which was started by two Harvard Business School graduates in 2010 , is an endurance event series which now takes place all over the world . Despite the rather unpleasant obstacles , the events have become so popular that many of them sell out weeks in advance . One participant at Sunday 's Tough Mudder in California explained why he had signed up for a second year running . 'It 's about being with your friends , pushing through . Everyone here is your friend , everyone 's your family . That 's all there is to it . Anyone can do it , ' he told Ruptly TV . At the end of the course , contestants are rewarded with a Tough Mudder sweatband and an ice cold beer . At most events , Tough Mudder also offers tattoos of its logo to all finishers who want to commemorate their achievement . The event founders highlight that Tough Mudder is a 'personal challenge ' and not a race , therefore no-one is timed and no medals are given out . However , participants generally finish the booby-trapped tracks in around three hours . The next Tough Mudder events are set to take place on May 2 and 3 in London , Texas and Atlanta . The average age of contributors is 29 , with 70 per cent male and 30 per cent female . New recruits : Tough Mudder , which was started by two Harvard Business School graduates in 2010 , is an endurance event series which now takes place all over the world . Swinging success : Despite the rather unpleasant obstacles , the events have become so popular that many of them sell out weeks in advance . Crawling low : The obstacles often play on common human fears , such as fire , water , electricity and heights . Soaked : A man climbs out of an icy water tank during the Tough Mudder 10-12 mile obstacle challenge in California on Sunday . Still afloat : An average 78per cent of Tough Mudder entrants successfully complete the course . Following the leader : The first Tough Mudder challenge was held in the U.S. in 2010 - to date , more than 1.3 million people worldwide have participated in the events . Tearful expression : A man scrambles through tear gas during the Tough Mudder 10-12 mile obstacle challenge in San Bernardino , California . Determination : At the end of the course , contestants are rewarded with a Tough Mudder sweat band and an ice cold beer . A messy affair : Most participants end up ditching their clothes around the course as they get coated with mud and sweat . Ready for more : The next Tough Mudder events are set to take place on May 2 and 3 simultaneously in London , Texas and Atlanta . Source of support : A couple share a passionate embrace after navigating a tunnel of barbed wire .
Tough Mudder , which was started by two Harvard Business School graduates in 2010 , is an endurance event series . On Sunday thousands flocked to San Bernardino in Texas to take on the challenge . The next Tough Mudder events are set to take place on May 2 and 3 in London , California and Atlanta . The average age of contributors is 29 with 70 per cent male and 30 per cent female .