Much like air conditioners falling from the sky, exploding manholes are one of those rare, unsettling and largely seasonal occurrences capable of instilling fear into even the most hard-boiled New Yorkers.

But unlike getting clobbered on the head with a window unit, a manhole erupting into flames and sending its 100-plus pound cast-iron cover sailing through the air doesn’t exactly qualify as a “freak” incident. It’s rare but not that rare — significant enough to warrant coverage on the evening news when it does happen; a big enough deal to prompt New Yorkers to eye manhole covers as potential projectiles each time they step onto the street.

When manholes do act up in New York, it’s usually, but not always, during the wintertime, often following heavy snowfall. Over the past several days, there have been over 200 “manhole incidents” reported to New York City’s electric utility, Con Edison. Most have been relatively minor involving a bit of smoke and plenty of rattled nerves.

While the frequency of manhole incidents isn't at all unusual for this time of year, two of the incidents, both in the otherwise placid Park Slope section of Brooklyn, have resulted in rare and violent explosions.

A 71-year-old Park Slope resident, out walking his dog near Prospect Park when the first of the explosions hit late in the morning on Feb. 2, was seriously injured when the unthinkable happened: he was knocked unconscious by a 70-pound flying manhole cover after it was catapulted 50 feet into the air. Following the explosion, Grillo's terrified dog, a black Lab named Abby, bolted into the park and didn't reappear until later that day when she wandered into a pharmacy located over 2 miles away from the explosion site near Prospect Park West and 4th Street. An animal rescue group was able to trace the disoriented pooch back to the Grillo family, thanks to her microchip.

An elderly woman was also injured when the windows in her apartment imploded during the same blast.

The second Park Slope manhole explosion occurred less than 24 hours later, early in the morning on Feb 3. Although no one was injured, six buildings were evacuated due to high carbon monoxide readings in the area brought on by thick, toxic smoke. A car parked directly over the exploding manhole was also completely destroyed.

So why, exactly, is there an uptick in manhole mayhem during the coldest months of the year?

The culprit is a relatively obvious one (and, no, it's not C.H.U.D):


However, snow alone can’t take the blame for the cold weather chaos. When snow melts and mixes with the thousands upon thousands pounds of salt used to prevent city streets from icing over, the resulting runoff seeps underground where it wreaks havoc on the vast network of subterranean electrical cables tucked away underneath the streets.

As noted by the Village Voice in a recent article that explores the phenomenon, corrosion brought on by salty and slushy roadways doesn’t just impact aging electrical wiring that's already in an iffy state. Relatively new electrical lines, free of rat nibbles and the normal wear-and-tear caused by traffic vibrations and other factors, can also erode when it comes in contact with a large amount of salt-heavy runoff.

The explosions themselves are the result of flame-ignited gas that's trapped within the tiny, electrical equipment-packed underground chambers. When build-up of the flammable gas becomes too great, the pressure can dislodge manhole covers, some weighing as much as 300 pounds, and send them sailing through the air like the world’s most dangerous just-popped champagne cork.

Although an investigation is underway to get to the bottom of what caused the explosion that rocked Park Slope on Feb 2., Con Edison spokesperson Bob McGee explains to the New York Times that the dreaded salt-and-snow combo was likely responsible: “If there’s any sort of fissure or crack in a cable, and salt gets into it, it creates mayhem. We’ve got a pretty good indication that this is what happened, because it’s in such close proximity to a storm where there was a lot of salt put down on the streets.”

Reports of underground fires tend to die down when the city is in the throes of a deep freeze. They pick up again when things begin to thaw out.

In an effort to minimize the risk of manhole covers turning into missiles, Con Edison has begun replacing solid manhole covers — there are nearly 300,000 of them spread out across the five boroughs — with vented ones that allow flammable gases to escape if the underground wiring catches fire.

While the vented manhole cover approach does help to prevent explosions caused by trapped gas and smoke, it doesn’t stop the thing that causes the explosions in the first place: salty, slushy runoff making its way underground. And as CBS News notes, it’s a far from foolproof solution as the first of the manhole explosions in Park Slope involved a new-model vented manhole. 

While the recent explosions have caused many New Yorkers to view manhole covers in a wary new light, it's business as usual for others. "I’ve been here all my life, and if anything, I guess I feared getting mugged growing up,” Tom Santisi, a Park Slope resident who lives just blocks from the Feb. 2 manhole explosion, tells the New York Times.

Via [NYT], [CBS], [The Village Voice]

Related on MNN:

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.