Linda Chamberlain works just down the hallway from her husband. She walks past him every day. Occasionally she'll stop by to check in on him and say hello.

The only problem is, Fred Chamberlain has been dead for eight years. Shortly after he was pronounced legally dead from prostate cancer, Fred was cryopreserved -- his body was filled with a medical-grade antifreeze, cooled to minus 196 degrees Celsius and carefully lowered into a giant vat of liquid nitrogen.

So when Linda visits Fred, she talks to him through the insulated, stainless-steel wall of a 10-foot-tall preservation chamber. And he's not alone in there. Eight people reside in that massive cylinder along with him, and more than 170 are preserved in similar chambers in the same room. All of them elected to have their bodies stored in subzero temperatures, to await a future when they could be brought back to life. Cryonically preserved in the middle of the Arizona desert.

This story is part of Hacking the Apocalypse, CNET's documentary series on the tech saving us from the end of the world.

Linda Chamberlain is cheerful as she shows me her husband's perhaps-not-final resting place. She places her hand on the cool steel and gives it a loving pat. Being in a room with 170 dead people isn't morbid to her.

"It makes me feel happy," she says. "Because I know that they have the potential to be restored to life and health. And I have the potential of being with them again."

Alcor proclaims itself a world leader in cryonics, offering customers the chance to preserve their bodies indefinitely, until they can be restored to full health and function through medical discoveries that have yet to be made. For the low price of $220,000, Alcor is selling the chance to live a second life.

It's a slim chance.

Critics say cryonics is a pipe dream, no different from age-old chimeras like the fountain of youth. Scientists say there's no way to adequately preserve a human body or brain, and that the promise of bringing a dead brain back to life is thousands of years away.

But Alcor is still selling that chance. And ever since Linda and Fred Chamberlain founded the Alcor Life Extension Foundation back in 1972, Linda has watched Alcor's membership swell with more people wanting to take that chance. More than 1,300 people have now signed up to have their bodies sent to Alcor instead of the graveyard.

And when her time is up, Linda Chamberlain plans to join them.

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Photographs of "patients" line the walls of Alcor's offices.

From the outside, Alcor's facilities don't look like the kind of place you'd come to live forever.

When I arrived at the company's headquarters, a nondescript office block in Scottsdale, Arizona, a short drive out of Phoenix, I expected something grander. After all, this is a place that's attempting to answer the question at the heart of human existence: Can we cheat death?

I've come here to find out why someone would choose cryonics. What drives someone to reject the natural order of life and death, and embrace an end that's seen by many, scientists and lay people alike, as the stuff of science fiction?

But after a short time at Alcor, I realize the true believers here don't see cryonics as a way to cheat death. They don't even see death as the end.

"Legal death only really means that your heart and your lungs have stopped functioning without intervention," Linda Chamberlain tells me. "It doesn't mean your cells are dead, it doesn't mean even your organs are dead."

Alcor refers to the people preserved in its facilities as "patients" for that very reason -- it doesn't consider them to be dead.

In Chamberlain's view, the idea of death as an "on-off switch" is outdated. People that died 100 years ago could well have been saved by modern medical interventions that we take for granted in the 21st century. So what about 100 years from now? Alcor hopes that by pressing pause on life, its patients might be revived when medical technology has improved.

"Our best estimates are that within 50 to 100 years, we will have the medical technologies needed to restore our patients to health and function," says Chamberlain.

We're killing people who could potentially be preserved. We're just throwing them in the ground so they can be eaten by worms and bacteria.

Alcor CEO Max More

Alcor CEO Max More agrees. In his view, cryonics is about giving people who die today a second chance. And he says our current views about death and burial are robbing people of a potential future.

"We're killing people who could potentially be preserved," More says. "We're just throwing them in the ground so they can be eaten by worms and bacteria, or we're burning them up. And to me, that's kind of crazy when we could give them a chance if they want it.

"If you think about life insurance, it's actually death insurance -- it pays out on death. This really is life insurance. It's a backup plan."

An early copy of Cryonics magazine sits in Alcor's offices, showing the inside of one of its preservation chambers.

Alcor hasn't exactly mapped out how its patients will be brought back to full function and health, or what revival technologies the future will bring. Its website speaks about the possibility of molecular nanotechnology -- that is, using microscopic nano-robots to "replace old damaged chromosomes with new ones in every cell."

But that level of cellular regeneration isn't something Alcor is working on. The company is in the business of selling preservation, but it's not developing the technologies for restoration. In fact, no one currently working at Alcor is likely to be responsible for reviving patients. That responsibility will be handed on to the next generation (and potentially many more generations after that) -- scientists of some undetermined time in the future, who will have developed the technology necessary to reverse the work that Alcor is doing now. It seems like a convenient gap for cryonics: Sell the promise in the present without the burden of proving the end result.

Our goal is to have reversible suspended animation, just like in the movies. We want it to be that perfect.

Alcor founder Linda Chamberlain

Chamberlain herself admits the future is ultimately unclear and that they "don't know how powerful the revival technologies are going to be." But she does know the end result Alcor is aiming for.

"Our goal is to have reversible suspended animation, just like in the movies," she says. "We want it to be that perfect. We're not there yet, but we're always working on improving our techniques."

The science behind cryonics is unproven. The procedures are highly experimental. No human -- specifically, no human brain -- has been brought back from death or from a state of postmortem preservation. Alcor points to research in worms and the organs of small mammals that it says indicates the potential for cryonics. There are famous names associated with the movement (Alcor admits famed baseballer Ted Williams is a patient), but there aren't exactly any human success stories who've awoken from cryonic preservation to hit the motivational speaking circuit.

James Bedford, the first man to enter cryonic suspension, according to Alcor. Bedford was preserved in a "cryocapsule" in 1967 (five years before Alcor was founded), before being transferred into Alcor's facilities in 1991.

Even More isn't making any promises. He acknowledges that the company may not even exist when it comes time for its patients to wake up.

"There are no guarantees," he says. "We're not promising to bring you back on May 27th, 2082, or whatever. We don't know officially this will work. We don't know for sure that the organization [Alcor] will survive... We don't know if an asteroid will land on us. There's no guarantees. But it's a shot. It's an opportunity. And it just seems to be better than the alternative."

The way the Alcor team sees it, you have a better chance of waking up from here than you do if you're sent to the crematorium.

One of the central questions of cryonics is how you preserve a dead body if you hope to revive it.

Even if they don't know exactly when or how patients will be brought back, the team at Alcor knows one thing is vital: They need to preserve as much of the brain and body as perfectly as possible.

While they may be clinically dead when they arrive in the operating room, Alcor's "patients" are intubated and kept on ice while a mechanical thumper (shown here on a dummy) keeps blood flowing around the body, all in a bid to preserve the body as thoroughly as possible.

That life-saving mortuary practice takes place inside Alcor's operating room -- a sort of hospital-meets-morgue where the organization prepares bodies for "long-term care."

When patients come through the doors at Alcor, they've already been pronounced legally dead. Ideally, they haven't had to travel far to get here and they've had their body put on ice as soon as possible after clinical death. According to Chamberlain, that hypothermia is vital for "slowing down the dying process." I didn't think I'd hear someone say that about a dead person.

During the first stages of cryonic preservation, bodies are "perfused" with a medical-grade antifreeze, all in a bid to prevent ice crystals forming. From here, the body vitrifies, rather than freezing.

(I also didn't expect to see a dead person in the operating room. At least, that's what I thought when I saw a human dummy waiting in the ice bath by the door. One of Alcor's employees picked up the dummy's hand to wave at me and I genuinely think that moment shortened my life span by two years.)

The ice bath is the first step in the preservation process, and it's here where the patient is placed in a kind of post-death life support. Drugs are administered to slow down metabolic processes, the body is intubated to maintain oxygen levels, and a mechanical thumper pumps the heart to ensure blood keeps flowing around the body.

The team then prepares the body to be cooled down to its permanent storage temperature. The blood is replaced with cryoprotectant (think of it like medical-grade antifreeze), which is pumped through the veins, all in a bid to (surprisingly) prevent the body freezing.

Freezing might sound like the natural end goal of cryopreservation, but it's actually incredibly damaging. Our bodies are made up of about 50 to 60% water, and when this water starts to freeze, it forms ice crystals which damage the body's organs and veins.

But if that water is replaced with cryoprotectant, Alcor says it can slowly reduce temperatures so the body vitrifies -- turning into a kind of glass-like state, rather than freezing. From here, the body is placed in a giant stainless steel chamber, known as a dewar. And Alcor says a cryopreserved body can be stored in this "long-term care" for decades.

I missed something when I first walked into the operating room. At the back, behind the ice bath and medical instruments (including surgical scissors and, chillingly, unexplained saws), there's a clear box, about the size of a milk crate, with a circular metal ring clamped inside.

It's a box for human heads.

This is designed for patients who've elected to preserve their head only, removed from the body from the collarbone up. These preserved heads are referred to as "neuro patients."

This small perspex box in the Alcor operating room is used to clamp human heads in place for cryopreservation.

If putting my whole body on ice was a bridge too far, then cutting off and preserving my head is beyond anything I can fathom. But it's a choice some of Alcor's patients make. The neuro patients are stored in small, barrel-sized vats while they wait for long-term care. The moment I lifted the lid on one of these vats -- nitrogen gas billowing out, human head obscured just inches below -- will stay with me forever.

Each preservation chamber can hold four bodies (positioned with the head at the bottom, to keep the brain as cool as possible) and five "neuro patients" stacked down the center.

It's cheaper if you elect to preserve just your head. Alcor charges only $80,000 for the head, compared with $220,000 for the full body. But there are also pragmatic reasons for choosing this more selective form of cryonic preservation.

When Alcor cryopreserves a body, the main priority is to preserve the brain and cause as little damage as possible. After all, the brain is not only the center of cognitive function, but also long-term memory. Essentially everything that makes you who you are.

You might be attached to your body now (both figuratively and literally), but many people at Alcor believe that, by the time medical science has advanced enough to bring a person back to life, their full body won't be needed. Whether you're regenerating a human body from DNA found in the head or uploading a person's consciousness to a new physical body, if we reach a point where cryonic preservation can be reversed, potentially hundreds of years in the future, your 20th or 21st century body will be outdated hardware.

That's certainly a view Linda Chamberlain takes. When she goes, only her head will stay.

"There's a lot of DNA in all that tissue and material," she says of the human head. "A new body can be grown for you from your own DNA. It's just a new, beautiful body that hasn't aged and hasn't had damage from disease."

In fact, when Chamberlain thinks of her future body, she doesn't want to limit herself to the kind of human form she has now.

"I hope that I won't have a biological body, but I'll have a body made out of nanobots," she tells me. "I can be as beautiful as I want to be. I won't be old anymore."

I hope that I won't have a biological body, but I'll have a body made out of nanobots.

Alcor founder Linda Chamberlain

I tell her she's already beautiful. She laughs.

"But if you have a nanobot swarm, it can reconfigure itself any way you want!" she replies, completely serious. "If I want to go swimming in the ocean, I have to worry about sharks. But after I have my nanobots body, if I want to go swimming in the ocean, I can just reconfigure myself to be like an orca, a killer whale. And then the sharks have to look out for me."

Waking up 100 years from now as a fully reconfigurable, shark-hunting nanobot orca sounds like fun.

But this kind of future is possible only if the process of going into cryonic preservation doesn't damage your brain. The brain is a staggeringly complex organ, and storing it at subzero temperatures for decades at a time has the potential to cause serious cellular damage.

And according to some scientists, that's the main issue with cryonics. Before you even get to the issue of reanimation, they say, cryonics doesn't come close to delivering on the promise of preservation.

Surgical instruments in Alcor's operating room.

Neuroscientist Ken Hayworth is one expert who's highly skeptical. Hayworth isn't opposed to preservation -- he was a member of Alcor before he left to found the Brain Preservation Foundation with the goal of building dialogue between cryonicists and the broader scientific community. He wants brain preservation to be a respected field of scientific study. And in 2010, he laid down a challenge to help build that credibility.

"[We] put out a very concrete challenge that said, 'Hey, cryonics community, prove to us that you can at least preserve those structures of the brain that neuroscience knows are critical to long-term memory, meaning the synaptic connectivity of the brain," he says.

"The cryonics community, unfortunately, has not met the bare minimum requirements of that prize."

Hayworth says he's seen examples of animal brains preserved using techniques very similar to what cryonics companies say they use, but the samples showed a significant number of dead cells.

"I take that to mean that there was probably a lot of damage to those structures that encode memory," he says. "It was like, 'We're looking at something that doesn't look right at all.'"

We're looking at something that doesn't look right at all.

Ken Hayworth

However, Hayworth has seen a technique that successfully preserved a brain so well that it was awarded the Brain Preservation Prizeby his foundation. This prize recognized a team of researchers for preserving synapses across the whole brain of a pig. But the technique, known as "aldehyde stabilized cryopreservation," has two limitations that differ from the promise of cryonics. Firstly, it requires the brain to be filled with gluteraldehyde, a kind of embalming fluid, which means the brain can never be revived. And secondly? It's a lethal process that needs to be conducted while a mammal is living.

"It almost instantly glues together all the proteins in the brain," says Hayworth. "Now you're as dead as a rock at that point. You ain't coming back. But the advantage of that is it glues all of them in position, it doesn't destroy information."

Retaining that information is vital because, according to Hayworth, it could allow you to re-create a person's mind in the future. Forget transplanting your head onto a new body. Hayworth says the information from a preserved brain could potentially be scanned and uploaded into another space, such as a computer, allowing you to live on as a simulation.

You might not be a walking, talking human like you once were. But, in Hayworth's view, that's not the only way to live again.

"I think there's plenty of reason to suspect that future technologies will be able to bring somebody back -- future technologies like brain scanning, and mind uploading and brain simulation."

Being preserved long enough (and well enough) that you can live on as a simulation may be one of the end goals that cryonicists hope to achieve.

But there are plenty of critics who say we won't reach that point anytime soon. They say there's no way to know whether cryonics adequately preserves the brain, because we don't fully understand how the mind works, let alone how to physically preserve its complexity.

Ken Miller is a professor of neuroscience and co-director of the Center for Theoretical Neuroscience at Columbia University in New York. He's spent his life trying to understand the complexity of the human brain.

"Some people say [the brain] is the most complicated thing in the universe," says Miller.

"The most basic answer to how the brain works is, we don't know. We know how a lot of pieces work ... but we're very far from understanding the system."

It's at least thousands of years before we would know and really understand how the brain works.

Ken Miller

According to Miller, while we know a lot about parts of the brain -- how the neurons function, how electrical signals travel to the brain -- the complete picture is still a mystery.

"In my opinion, it's at least thousands of years before we would know and really understand how the brain works to the point where you could take all the pieces ... and put it back together and make a mind out of it," says Miller.

"It's just the complexity. Levels and levels and levels and levels -- it's beyond the imagination."

And what if we reach that point? What if, a thousand years from now, science was capable of restoring my cryonically preserved brain and uploading it to some kind of simulator -- would I still be me?

Sitting in his office, I put the question to Miller. And in the kind of meta way that I've realized is normal when speaking to a professor of theoretical neuroscience, I see the cogs of his mind working. His brain, thinking about another brain, living on as a simulated brain. My brain is melting.

"I think so, but it's a funny question," he says. "Because of course, if it was all information that you got up into a computer... making something feel like Claire, we could have a million of them on a million different machines. And each of them would feel like Claire.

"But immediately, just like twins -- immediately, identical twins start having divergent experiences and becoming different people. And so all the different Claires would immediately start having different experiences and becoming different Claires."

Back in Arizona, with the vision of a million computerized versions of myself enslaving the human race far from my mind, the promise of cryonics still feels like a dream.

I'm walking through the long-term care room as waterfalls of fog cascade from the cryonic chambers. These dewars need to be regularly refilled with liquid nitrogen to make sure patients stay at the perfect temperature, and today's the day they're getting topped up.

As I slowly step through the fog, stainless steel chambers loom large around me. Visibility drops, so I can barely see my outstretched hand in front of my face. For just the tiniest moment, as my feet disappear beneath me and I'm surrounded by reflections on reflections of white vapor, I lose my bearings. I feel like I'm having an out-of-body experience.

Walking through Alcor's long-term preservation room is a surreal experience.

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