There's a judicial no-man's land in a beautiful part of the USA, where a murderer could get off scot-free, says a law professor.
Imagine a place so remote, you can't access it by road. There's no sign of humans, you're in the wilderness, and you're on top of a volcanic hotspot.
You might say it's the perfect place to kill someone. As an added bonus, you might even get away with it - thanks to a bizarre legal loophole.
WHERE IS IT?
Welcome to Yellowstone, one of America's largest national parks. It's an area of nearly 9000 square kilometres, straddling three states: Wyoming, Idaho and Montana. It's also home to the mysterious 'Zone of Death'.
The region first made global headlines in 2007, thanks to a thriller called Free Fire by C.J. Box. His novel is based on the premise that an 80 square kilometre section of the park is beyond the reach of the law.
The concept was first suggested by Michigan State University law professor Brian Kalt in 2005: he describes it as a judicial no-man's land, where a murderer could get off scot-free due to sloppy lawmaking.
"The more I dug into it, the more interested I got," he told Vice.
"People have this fascination with uncovering a loophole for the perfect crime. There are a lot of different approaches to it. But in terms of geography, there's just this one spot."
HOW DOES IT WORK?
Even though Yellowstone is in three states, the whole thing falls into the federal district of Wyoming. The legal technicalities are pretty dry, but a Vox article summarised it like this:
Say two people are camping in the Idaho part of Yellowstone. They get into a fight and one kills the other - but rather than cover it up, he admits it.
He's entitled to be tried by jury. According to the Sixth Amendment, the jury should be people from the state (Idaho) and federal district (Wyoming) where the murder was committed. But no-one lives in the Idaho part, so it's impossible to get a jury.
Since he can't be guaranteed a fair trial, he walks free.
ARE THERE EXCEPTIONS?
The crime needs to be bad enough to warrant a jury. Lesser offences are resolved by a judge, who is perfectly entitled to impose fines or even short jail sentences.
Also, the whole thing has to happen in the park, which means the decision to murder someone must be spontaneous - so don't get any ideas.
"If it were orchestrated elsewhere, the defendant could be charged with something like 'conspiracy to commit murder' in another district," Jacob Baynham wrote in Vice.
HAS IT EVER BEEN TESTED?
The Zone of Death was put to the test once, in 2005, when a hunter illegally shot an elk.
He was standing in the Montana part of the park when he pulled the trigger, but he was indicted in Wyoming. He successfully argued he had a right to be tried by jurors from Montana, and while people do live in that section, there are too few residents to form a legitimate jury.
"The court dismissed the argument out of hand, precisely because it would imply that Yellowstone contained a Zone of Death," wrote Dylan Matthews.
WHY ISN'T ANYONE FIXING IT?
When Professor Kalt first published his paper 'The Perfect Crime', he was worried it could inspire someone to commit murder. He sent advance copies to a variety of government departments, trying to close the loophole before he told the world about it, and he even drafted the legislation for them. However, no-one cared.
"Nothing happens in Washington just because it's a good idea," he told Vice.
"If Congress really wanted to fix this, it wouldn't take long at all. The problem isn't that it's complicated; it's that they're not interested in it."
"I feel like I've done what I can to prevent this; the blood will be on the government's hands," Kalt told Vox in December 2014.
Eleven years since he brought the issue to light, the loophole still exists.
- additional editing, Eveline Harvey