Factors including age, pressure, and weather can contribute to a main's breaking. Breaks, in fact, are a daily occurrence in a system where the smallest mains are six inches in diameter, the largest 93. But most breaks are so small that the general public doesn't notice - until one like the 48-incher below Frankford and Torresdale bursts, disrupting a swath of the city.
About 23 million gallons gushed from that 106-year-old main Monday, flooding streets and businesses, and closing nearby schools.
A cause has not yet been determined. In fact, by Friday afternoon, excavation crews had still not reached the pipe, which is 20 feet below ground. Once they do, they will remove the broken piece and send it to a lab for analysis, said Water Department spokeswoman Laura Copeland.
A similar 48-inch pipe that ruptured at 21st and Bainbridge Streets in July 2012 broke as a result of external pressure "being exerted by above-ground infrastructure, which, over time, stressed the pipe to the point of failure," city officials said.
That pipe's age: 126. The damage to homes and businesses: $2.8 million in claims submitted to the city. Claims from Monday's incident might hit similar heights - three car dealerships in the area, for example, reported the loss of many vehicles.
Can such devastating breaks be prevented?
The answer lies in Miller's flat-tire analogy. A pipe can be fine, but the right amount of pressure at the right angle can cause a catastrophic break.
"Being underground puts them at the mercy of above pressures," Miller said.
Then there's age. Seventy percent of the city's water infrastructure is cast-iron pipes, many more than a century old. As pipes are replaced, a ductile iron material, thought to be more flexible and sturdier, is being used.
To replace all 3,000-plus miles of pipe, however, would cost $4 billion, according to Water Department estimates.
The city has already spent a half-billion dollars to replace 18 percent of its pipes over the last 32 years. It is committing to spending $34 million annually, or $204 million over the next six years, for its water-main replacement program, Copeland said.
"There is a concern as our infrastructure ages," Copeland wrote in an e-mail. "However, it is a delicate balance to keep rates affordable for our customers."
By July, water rates will have increased 17.5 percent since 2012. Part of the additional revenue goes toward replacing pipes.
Maintaining the aging system also means adding nonharmful chemicals to the water and lining the inside of large cast-iron mains with cement, both to reduce corrosion.
California has tried PVC pipe, which is cheaper than ductile iron, but the city's largest pipes would not fare well with the material, Miller said.
"A 48-inch one would collapse," he said.
Because of the high cost of replacing mains, the city tries to time its scheduled replacements to coincide with other road or underground work in the vicinity.
The goal is to replace 22 miles of mains each year. In fiscal 2008, the most recent with a total available from the Water Department, the city replaced 20 miles of mains.
But those are the planned repairs. Emergency repairs or replacements such as the one at Frankford and Torresdale are much more costly, due to "triple bottom-line costs" such as loss of property, related injuries, and traffic disruptions, Water Commissioner Howard Neukrug said.
"Replacing or renewing a pipe before a break is always better than repairing after a break," he said in an e-mail.
To replace a main or just repair it is also a complicated decision.
"You don't want to replace a main until the end of its useful life," said R. Scott Hughes, a professional engineer at the Valley Forge firm Gannett Fleming. Hughes, who has 35 years of experience with water systems, said repairs often make more sense than replacement.
In Philadelphia, the vast majority of water-main breaks occur in smaller mains. Breaks average more than one per day.
Miller said older cities, such as Boston, New York, and Charleston, S.C. - "colonial cities," as he put it - face situations similar to Philadelphia's. But the climate is less forgiving in the North.
"Down south, they have less stress to their system because of the weather," Miller said.
In its 2013 infrastructure report, the American Society of Civil Engineers gave Pennsylvania's water infrastructure a grade of C-minus, saying the state will need to come up with $11.4 billion over the next 20 years to replace aging facilities.
But the Keystone State at least is a step above the national average.
The society gave the entire country a D-plus.