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.
"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."
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."