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."
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."
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.