finished ultramarathons in the US alone.

An ultramarathon is any race longer than 42 kilometers,

and races can range anywhere from 50

to over 4,000 kilometers.

But one of the more common lengths

covers about 100 kilometers

and takes on average 17 hours to finish.

In some of the longer races,

half the runners never see the finish line,

and that's because these races not only push you

to your mental limit, they take a toll on your body

both inside and out.

Ultrarunners endure some pretty extreme conditions.

The Badwater Ultramarathon, for example,

covers 4,000 meters of elevation through Death Valley

in July, one of the hottest months of the year.

Temperatures can reach up to 47 degrees Celsius,

causing headaches and dizziness in many runners.

But even if you're running in more comfortable weather,

you could still be feeling sick to your stomach.

One of the most common issues in any ultramarathon

is nausea or vomiting.

It affects about 37% of people who complete a race,

but it's also the No. 1 reason

runners won't finish in the first place.

That's because running disrupts your digestion.

It diverts blood away from your stomach to your muscles,

and since ultramarathons last so long,

many races provide sandwiches, pasta,

and other carb-loaded meals.

But as a result, that food will just sit around

undigested causing a lot of problems.

But if your stomach isn't slowing you down,

your eyes could be causing you grief.

Windy conditions can dry out or damage the cells

that pump a protective liquid layer

over your cornea, which can cause it to swell up

and blur your vision.

During the Hellgate 100K, for example,

runners report losing most of their vision

by the end of the race.

They call it Hellgate Eyes.

But no amount of protection or preparation

can prevent one symptom: That's sleepiness.

For a 17-hour race, it might not be so bad,

but longer races can stretch for more than 24 hours,

which can make sleepy runners more prone to trips and falls

or worse: hallucinations.

The Barkley Marathons, for example,

is a race where runners are given 60 hours

to finish 161 kilometers.

In 2005, one runner reported that he thought he saw houses

at the top of the mountain

and believed he was their garbage man

sent to pick up their trash.

Despite these health hazards,

ultrarunning is a growing sport, believe it or not.

One study found that compared to marathoners,

ultrarunners are more likely to do it for nature

and a sense of purpose than for the competitive aspect.

So if you're up for the challenge,

plenty of races are on trails through natural areas.

So at least you'll get a great view.

You might be hallucinating

that you're someone's garbage man, but it will look pretty.