Sept. 23, 2009 -- Millions of tourists walk up and down the Las Vegas strip every year, looking to have fun and make some money. But beneath the flashing lights, there is a much darker side of Las Vegas.

Underneath Sin City's most famous casinos is a secret world: a labyrinth of tunnels that run for miles under the Las Vegas Valley. Built to protect the desert city from flash floods, the tunnels have become home to hundreds of Las Vegas' homeless.

Nightline visited the underground world beneath the Las Vegas strip, with Matthew O'Brien, author of "Beneath the Neon: Life and Death in the Tunnels of Las Vegas," as our guide.

"Even after exploring these tunnels for seven years, you still have a bit of anxiety when you're walking into the tunnels because you never know what you're going to find," O'Brien said. "You never know what's waiting in the dark."

O'Brien introduced us to Iron, a tunnel regular, who's made a makeshift home in a tunnel with a view of the strip that some hotel guests pay top dollar for.

"That's pretty much it, just a blanket and some pillows right now, because everything I had got washed away," he said as he led us into his tunnel, so low overhead that we had to crouch. "It's kind of dirty, I wasn't expecting company."

When he become homeless seven years ago, Iron said moving into the tunnels took some coaxing.

"It took them months to get me into these tunnels; I used to be scared to death of these tunnels, I wouldn't come in here," Iron said. "Finally I came in, but at the beginning, I wouldn't go no further than this. Now I'll go all the way in."

For Iron and other city homeless, the tunnels provide refuge from the blazing summer heat.

"You're going to get the shade. It's cooler in here," Iron said, "Over the summer, it was 115 degrees, it's 15 degrees cooler in here."

The Dark Underside of Las Vegas

But there are real dangers. Rain waters can fill them up with more than a foot of water per minute, washing away camps like Iron's.

"They can be very dangerous, said O'Brien. "A lot of [the dwellers] are really good about communicating with each other about when it's about to rain, so they can just grab their valuables and get out, and leave everything else behind."

The tunnels have been a refuge for convicted criminals, like Timmy T.J. Weber, who used them in the summer of 2002 to evade a massive police manhunt for weeks. It was Weber's story that brought O'Brien to explore the tunnels.

"That got me interested in what [Weber] heard in these tunnels, what he smelled, what he saw. And so I followed his path, and then I continued to explore other tunnels," O'Brien said. "We were not expecting to find people ... That was just not something that was on our mind. And when we stumbled upon the first camp, when you see the silhouette of a bed and a makeshift grill and a person, it's a strange experience."

Exploring the Tunnels' Underworld

A half mile into a tunnel that runs beneath Caesar's Palace -- a major landmark on the strip -- it was eerily quiet, even though thousands of people filled the streets above.

Amidst the darkness, holes in the walls are designed to level off flood waters, but also serve as doors, O'Brien explained. Areas are designated for garbage, a makeshift bathroom -- and even graffiti art.

As O'Brien led the way, it wasn't long before we ran into Steve, who was riding his bike through the pitch-black tunnel that he calls home. He's a Las Vegas native who moved underground two years after a drug problem left him homeless. He guided us to the spot in the tunnel he shares with his fiancé Katherine, who moved in with him a year ago, after he agreed to stop using drugs.

"We fell in love…We want to get out of here, that's always our main goal. We don't want to live like this forever," he said. "We don't like living in the tunnels. We live here because we can, and we're not bothered by anyone. The police and people like that don't really know that we're down here. You know, it's a lot of out-of-sight, out-of-mind."

The couple has worked hard to make it as homey as possible.

"Just because we live outside doesn't mean we can't be comfortable," Steve said, showing off a makeshift shower he'd crafted from a water jug and a spout. "Our shower...works just fine…Need a little privacy, just put the curtain up."

Steve introduced us to some of their neighbors, like Phil, who has also battled addiction. He told us he lost his job about a year and half ago, when the economy started to sour.

Sleep by Day, Casino Slots by Night

"I worked at the Onyx, which was behind Hooters casino. It was a condominium structure, and that was four years. So, I had four years of actually being off the street and in an apartment. So that was rather nice," he said. "And then once the economy got bad, there was no work. Unemployment didn't last forever, so I ended up back here."

When we met Phil, he was reading an issue of "Sports Illustrated," catching up on the latest scores, to place his bets -- in hopes of winning big in one of the casinos above him.

"I bet sports, thinking that I'm going to hit it rich and get out of this situation," Phil said, "but everyone who comes to Vegas thinks the same thing I'm thinking."

Many tunnel dwellers survive by venturing out to the casinos at night to scour the slot machines for credits tourists left behind -- a practice known as credit-hustling.

"There's lots of money in the casinos. All you have to do is just keep your eyes open," Steve said. "You can find money on the ground. People leave money in the slot machines without realizing it. ...I don't mind taking money from those who live a life of decadence."

$50 a Day

Sleeping by day and playing the slots by night, Steve and Katherine said they earn about $50 a day or more, which is enough to live -- even splurge for a movie once in a while.

"When you're living down here, it can be bleak -- very bleak. It's easy to come back here and think, just give me something. Just get high," Steve said. "That's why it's very helpful to have someone with you that you really love, through thick and thin, in tough times, because it hasn't been easy, not at all."

While they've made the tunnel their home, the couple said they'd move out tomorrow if they could.

"We don't plan a month ahead of time or whatever. Right now, we're living day to day. We get up, we have to survive. So it's hard for us to really look to the future."

It's all about survival -- trying to keep the darkness at bay, one day at a time.

"I think you'll find down here, just because it's dark, no electricity," said Katherine, "time kind of stands still."