When a 20-inch steam pipe buried 15 feet under the streets of St. Louis burst two weeks ago, most people focused on the potential health hazards in what was blown skyward.

But the blast got St. Louis Public Radio’s Rachel Lippmann thinking - what’s under the streets that we don’t know about until it’s revealed?

This is what she found out.

So, what's under the streets?

The intersection of 11th St. and Washington Ave., not too far from where the steam pipe burst, looks like any number of intersections in the city of St. Louis, with street lights, fire hydrants, parking meters, all sorts of things aboveground. But that's only what's visible. Here are some of the answers you get when you ask people what's under the streets.

"My wild guess would probably be rats, rodents, probably snakes," says Dominic Johnson. "Judging from some of the smells coming out of these grates, I would say it's a lot of muck and stuff that's just overflow from the river," replies Seth Womack. "Sewers, and power lines, and probably rats and mice, and maybe cats and stray dogs," is the answer from Mike Webb.

Rich Bradley is the city's chief engineer, and the keeper of the street's secrets.

"It's any utility that you can imagine," he says. "Big sewer lines, water lines, the steam lines downtown, electric utilities, gas lines, street light lines, fiber optic lines. If you don't see a lot of wires on poles, it's usually because they're buried in the ground."

Bradley says crews also run into old trolley tracks, or brick and cobblestone streets laid down 100 years ago or more.

It all adds up to quite a tangle under the streets. Add in the fact that some of the utilities have been abandoned, and it's no wonder that crews who have to dig into the streets practice a version of the old adage, "measure twice, cut once."

"There's a lot of investigation before you even start digging," Bradley says. "Back in the day, when people did mapping of items, it was not very exact. The locations are off by quite a bit. Most of the time they would build all day, they would come home at night and they would write down what they thought they remembered they built. Today's it's a lot different. We can locate things plus or minus a quarter of an inch."

Bradley wants to set a few things straight. Yes, there are tunnels connecting City Hall to other government buildings downtown. But they're not escape hatches.

"They're very small, and most of the time they're filled three-quarters of the way up with water," Bradley says. "If it was an emergency, I would still take my chances above ground. I think people confuse the tunnels with the vast amount of caves that exist in St. Louis."

Caves beneath our feet

Yes, caves.

There's at least 30 of them, including one under Union Station and another that once ate the pond in Benton Park. Bradley's run into a few during construction.

"As we dug board for the drill caissons at the Scottrade Center parking garage, we ran into a couple areas where we ran into large rooms in the ground, which were rock formations, and had to fill those in order to build the building," he said.

Many of the caves were tourist attractions before being filled in by rubble. A number were central to early commerce.

One of the few that's still accessible - though not publicly -snakes for a half-mile or more 40 feet below the sprawling Lemp Brewery complex in south St. Louis, which has been owned since 2005 by Shashi Palamand.

"There are a lot of caves in Missouri, a lot of caves in the United States, a lot of caves in the world," he says. "But not that many were used for an industrial process, something that had a direct relationship to the economy, and to commerce."

Like most of the city's 19th-century brewers, Adam Lemp specifically purchased a property with access to a cave, which provided temperature-controlled space in the days before artificial refrigeration. Palamand says that as technology improved and the caves were filled, the Lemp family turned them into their own private playground, complete with a theater and a pool. But that's not the only thing that fascinates him about the caverns.

"There's actually a few animals that live down here - small, tiny creatures," he says. "We have fossils in these caves from ancient mammals and reptiles - I'm talking from 100 million, 200, 200 million years ago. They have an industrial history, they have a geological history, and they have a zoological history." (In 2005, the American Museum of Natural History wrote a paper on a deposit found in the Lemp caverns.)

From prehistoric village to modern metropolis

The roads in St. Louis cover more recent history too, says Joe Harl, an owner and lead investigator at the St. Louis Archeological Research Center.

"There's almost a 100 percent chance that the remains of the prehistoric village where St. Louis started a thousand years ago is still intact beneath the city," Harl says. "St. Louis was a large mound village, just like Cahokia at one time."

It's not uncommon, he says, to find human remains.

"People assume that when graves get moved, they move the bodies," he says. "Well, what happens is, they move the gravestones and leave the bodies behind. Every graveyard that we've ever known has been that way." He says his company tries to document as many of the sites as possible, but it's tougher now that Missouri state law has been interpreted to mean that only undertakers and the police have to be involved when remains are discovered - and that only happens if they're reported.

In Missouri, research is only required at the site of large federal projects, and Harl says that's a shame. Bodies aside, there are fascinating things under there - like a 1900s trash dump, a current Research Center project.

"That was the beginnings of the industrial age, and the beginning of the consumerism," he says. "You can see all these national brands that are starting at that time and being marketed nationally."

Harl says most people don't appreciate what's beneath their feet in that hidden world where past meets present.

"I think it's because it's just not known to them," he says. "Nobody talks about our prehistory or our history at all. But I've always found that when people learn about it, they're extremely fascinated by it. And what's even more important, there's all kinds of lessons we can learn from the past."

The city's Rich Bradley agrees.

"I think it's very similar to your house," he says. "You turn the faucet on and the water works. You plug something in and the electric is there. I don't think the common person understands how that all gets to where it is."