They call themselves 'drainers' or 'urban explorers,' truly an underground group of hidden hobbyists who venture into North Texas drainage pipes where storm water goes out... and where people are not supposed to go in.
Self-identified urban explorer Kirby Enoch estimates the ranks of active urban archaeologists in North Texas are between 10 and 40.
'It's pitch black; no light whatsoever,' he said. 'The only light you have is the light you bring in with you. You can hear the heartbeat in your chest. You can hear the bugs climbing on the walls.'
To say the least, it can be creepy.
In some cases, it can be illegal, too.
Of all the major cities we checked with in North Texas, none had laws specifically outlawing the practice, but all of them strongly cautioned against the hobby, citing the obvious danger.
As we hiked into a cavernous drain with Enoch and fellow urban explorer Mike Schreiber, they pointed out markings on the wall of a drain that outlined which connecting pipe we should take when we reached a fork in the system.
'One of our fellow explorers has come here and marked out the way, because this side is full of debris, so it's not safe,' Schreiber explained.
There have been documented reports from around the world of urban explorers being drowned when flash floods fill the almost-empty conduits within seconds.
Schreiber and Enoch said they've both had close calls when flooding has flushed them out. It has happened to them when thunderstorms broke out; when a hockey rink above ground was melted and suddenly drained; and when a water-intensive industrial process at a factory finished up and instantly purged hundreds of thousands of gallons of water down the drains.
A pipe can even fill up immediately if a nearby water main breaks.
'It can be dangerous,' Schreiber conceded. 'Things can happen.'
We trekked about a mile into a North Texas maze with the duo, when Enoch showed us a message he had spray-painted on the wall about two years ago:
SOMETIMES I WONDER WHY I LIKE THIS
We were wondering, too.
'It's a place where you can go and at every level feel like you are not part of the above world,' Enoch explained.
He and others who do this claim they find peace underground. They also find a lot of other stuff.
While we hiked with them, they discovered crab claws, small fish, gift cards, a driver license, a few unidentifiable artifacts, and a one dollar bill.
But Enoch said there is a much bigger payoff to exploring the drains.
'It's very much an artistic expression,' he said.
Deep down, these urban explorers say they've found a creative spark.
'I like fireworks and the way they leave trails and light,' Schreiber said, describing his preferred (and illegal) method of illuminating the concrete tubes to take long-exposure photographs. Over the years, Schreiber has become a subterranean shutterbug.
'I've got probably 30 or 40,000 photos of drains,' he boasted.
Enoch has become a prolific sub-surface photographer, too. The two men are amazed at how much they can manipulate the pitch black environment.
They're also fascinated by how much the environment alters itself. They point out several stalactites forming on the ceiling of the drains.
'This is one of the largest deposits in here,' Schreiber said, noting that the formations morph each time they go back.
'That's the kind of stuff that just makes it more interesting,' he said.
Still, they don't recommend this hobby to the uninitiated. And they never go alone, because as Enoch explains, too much can go wrong.
'And no one can hear you scream,' he said.
It's a very risky pastime, but these explorers of the underworld point out that just like the runoff they slosh through, they've always eventually found their way out.