The red, sandy mass of the Simpson Desert stretches across 68,100 square miles of Australia's Northern Territory, extending over state borders into Queensland and South Australia. The landscape is relentless. In parallel lines, from north to south, dunes stretch as far as the eye can see – some of their ridgelines continuing unbroken for 200km, some of their peaks soaring to 40m. There are no maintained roads, no escape routes and, with summer temperatures reaching up to 50C, no second chances for the unprepared. It took Samantha Gash four days to run across it.
"There's a huge sense of space in the Simpson," says Gash. "There were only two turns to be made in the whole 379km route. You can see for miles and that sort of environment puts a lot of things in life into perspective."
Since 2008, the 27-year-old lawyer has run ultramarathons all over the world (though her epic run across the Simpson was not an organised event). To qualify as an "ultra" a race has to be longer than a traditional marathon (26.22 miles), with the most common distances for single-stage races being 31 miles, 50 miles, 62 miles and 100 miles. Multi-stage races take this a step further. In 2010 Gash became the first female – and the youngest person – to complete the infamous Four Desert Series, racing 250km in four separate stages across the Atacama, Sahara and Gobi deserts and Antarctica.
Another desert race, the Marathon des Sables, a six-day, 243km race across the Sahara Desert in southern Morocco, is considered by some to be the toughest on Earth.
"I just love deserts. Running is a way of exploring the world: that's why I do it," says Gash. "Once you tackle something bigger than you've done before, you break down boundaries and that's so powerful. How can you deny yourself the opportunity to find out how strong you are?"
It's a question people are trying to find their personal answers to with growing fervour, as the bog-standard marathon loses its reputation as the pinnacle of running endeavour.
"I did my first ultra in 1995," says Rory Coleman, a performance coach specialising in training people for long-distance races. "Back then it was a few oddballs looking for a different challenge. Now I think ultras are seen as the new triathlons, and the ultra race scene is exploding."
Together with his wife – ultra racer Jen Salter – Coleman organises events around the UK. "We had 300 enter our Cardiff ultra this year," he says. "Next year we'll easily have 500."
Despite the growing interest, the organisation of ultras is still rather disparate, with independent races popping up all over the place, giving the sport a slightly amateurish feel, with camaraderie playing a large part. There's a relatively comprehensive list of global events at marathons.ahotu.com. Some of these, such as the Dukeries 40-miler, which took place in May are billed as a gentle introduction to ultras. Others, such as Whistler's Meet your Maker – held for the first time last week, with the winner finishing in eight hours, 31 minutes – make no bones about what they are: 50 miles of undulating singletrack alpine terrain. So if you really want to run across the US's national parks, there's an ultra for you. And if you fancy tackling 4,600m of altitude gain in Luxembourg's Little Switzerland, you're in luck this year.
"Running has seen tremendous growth in the past 20 years," says Topher Gaylord of Mountain Hardware, an outdoor equipment company that has turned its attention to ultras enthusiastically. "There's been a tenfold increase in trail events, and the events have seen a massive rise in participation because it's such a natural way to engage with the environment."
Last weekend Gaylord was one of 2,743 runners who took part in the North Face Ultra Trail du Mont Blanc (ultratrailmb.com) , a single-stage race that begins in Chamonix and takes in 100 miles of Alpine scenery, three countries, 400 summits and almost 10,000m of altitude gain. It was won by a British runner in 12 hours 32 mins. Support is minimal and the elements merciless – both this year and last the weather was so bad runners ended up covering part of the course as snow fell around them.
This epic race, where the looming presence of Mont Blanc is the only constant, has demolished champions. Gaylord, who has entered the event 10 times, says: "No matter how prepared you are, it's always a challenge and there's never a guaranteed outcome. But it teaches you to be calm. There's not a lot you can do about the elements, so you have to learn to be comfortable with whatever you get."
Another competitor, Rachael Hutchins, adds: "It doesn't get more scenic than Chamonix. I remember just before reaching the first checkpoint in the TDS last year (the Traces des Ducs des Savoie – a subsidiary and slightly shorter event under the UTMB umbrella), I rounded a bend to be faced with the most amazing view of a glacier I've ever seen. It was spectacular."
Trail-based, or off-road, ultras are becoming the norm, with runners often speaking of the enjoyment of being part of the landscape, of the feeling of adventure. But city-based challenges still have a part to play in this relatively embryonic sport.
"My all-time favourite is the 3100," says Abichal Sherrington, editor of Ultrarunning World magazine (ultrarunningworld.co.uk). This is the longest certified foot race in the world, with runners lapping a 0.5488-mile course round an extended city block in Queens, New York City, until they clock up 3,100 miles in 52 days. "It's as close to heaven as you can get. It's like meditation. You enter a different realm. It's not boring because you're completely focused on that moment, on what's going on inside your body. We live in these tiny worlds with all these boundaries. But there's so much more to life, so much to explore and these races can be key to that."
While pounding out 100 miles may seem like an activity reserved for elite athletes, competitors generally agree that the opposite is true: ultras are for everyday people, and they're not wired differently. What they share is a desire to push boundaries.
"For me it's definitely a physical test," says Hutchins. "The setting is usually mindblowing and I find it easy to switch off my brain."
The advice from those in the know is start small and allow yourself to develop a love for the sport and an appreciation of the outdoors, building up your distance slowly.
And it seems it isn't just runners who have the opportunity to take their sport to a higher level. Cycling events such as 24 hours of Exposure (sip-events.co.uk) and the Strathpuffer 24 (strathpuffer.co.uk|), both UK endurance mountain bike events, are becoming increasingly popular, while the Ultramarathon Cycling Association of America lists 116 events taking place this year, compared with 75 in 2011. Stage bike races such as South Africa's 800km Cape Epic or the 3,000-mile road cycling Race Across America offer slightly more challenge for your buck.
"We are absolutely not different from anyone else," says Andrew James, who in 2009 broke the record for the UK's Ridgeway Challenge, running 85 miles in 12½ hours. "People of all ages and sizes can and always have covered large distances. I'm constantly bewildered by how my body can run so fast for so long."
