Let us tour Philadelphia's latest, most discussed sightseeing sensation.

"When something like this happens, it reveals what allowed this city to exist in the first place," explains historian Adam Levine, based on 14 years of expertise. "This is what makes us a first-world country."

Levine speaks not of the Barnes Foundation, mind you, but the literal foundation beneath the street. That would be the massive hole at 21st and Bainbridge, 20 feet deep and now 40 feet in diameter, created when a water main burst two weeks ago on a Sunday night, flooding the streets and several basements.

It's the Grand Chasm of SoSo, as the neighborhood south of South is often called, technically accurate but descriptively inadequate for this charming, booming neighborhood. Besides, there's nothing so-so about this hole.

Day and night, the hole draws spectators, who marvel at the history and hugeness of it all, sometimes lingering for an hour at a time. The place is a gaping hole and a gaper delay, a way to pass the time, our Olympics of municipal utility construction.

The chasm is like Boston's Big Dig, except it wasn't dug. The hole blew up on its own. A forensics team will determine the cause. (CSI: PWD) The hole reveals what lies beneath - cobblestones, trolley tracks, and 100-year old, cast-iron, 48-inch pipes.

"The surface of the city is so blank," says Levine, a consulting historian for the Philadelphia Water Department. "We don't know what's under there."

Yes, the Water Department has a historian, albeit part time.

PWD also has a museum (the Fairmount Water Works Interpretive Center) and a permanent exhibit (in the basement of the Municipal Services Building), making it the most culturally refined of all city utilities and service departments. Besides, who would erect a museum about L&I?

Water Commissioner Howard Neukrug arrives at the site, checks in with the crew of eight men mending a section of 48-inch pipe. "I'm here once or twice a day," he says. After 34 years with the department, two as boss, Neukrug is as fascinated by the hole as civilians are. Like a curator, he identifies the provenance of various pipes and wires operated by virtually every entity known to bill mankind.

When was the last time the department had a huge water break? Neukrug shrugs, smiles, "Well, Wednesday at 6:56 a.m., Front and Tioga."

Breaks happen. Indeed, all the time. But these sorts of breaks occur only 3.5 times a year on the big (36-inch or larger) transmission pipes, and rarely this close together. The average age of the city's pipes is 67 years. "Ideally, you would like to fix the pipes a day before they break," Neukrug says. So this was, literally, a bad break for the water department and neighbors, though water was restored quickly because distribution pipes that direct water to buildings were unaffected.

The city maintains 3,100 miles of water-main pipe. In a good year, like the last one, there were 530 breaks. The previous year, 965. Almost all the breaks were on the smaller (8-inch or less) distribution pipes. Large pipes are more susceptible to breaks in the summer, small ones more vulnerable in cold weather. What distinguishes the Grand Chasm of SoSo is the neighborhood's density, affluence, and proximity to Center City, plus the fact that it wiped out an entire intersection.

"Considering all the breaks, that's about 500 feet of damage in 3,100 miles of water-main pipe," Neukrug tells me. "The average is 240 breaks per 1,000 miles of pipe."

Philadelphia was the first major American city to establish a municipal water system, in 1801, a watershed where Chestnut Street met the Schuylkill. The first pumping station was a neoclassical building designed by Benjamin Henry Latrobe at CentreSquare, torn down in 1830 and ultimately replaced by the decidedly less modest City Hall.

Clean water and independent sewers "make us a developing country, allowing us to live in relative health, because you can't live without clean water," Levine tells me. "Clean water is what distinguishes, even today, the first world from the third." Before 1801, residents dug wells and maintained outdoor privies, dirty water contaminating soil and compromising the drinking-water supply.

Neukrug hesitates to say when work will be done, when the Grand Chasm of SoSo will be no more, but hopes by Labor Day. He cannot praise the men working 12 hours daily on the site enough. Meanwhile, neighbors and folks who work in the neighborhood keep stopping by to gape. "This is my fourth or fifth visit," says resident Steve Rubin. "It's fascinating to see what's going on under the ground."

Levine tells me during the tour, "I was excited when I heard about this, because I've always been interested in infrastructure," but adds, "although I don't think the people in the neighborhood appreciate what happened."

True, but what other community can claim its own Grand Chasm?

Contact Karen Heller at 215-854-2586, kheller@phillynews.com, or follow on Twitter @kheller.