What causes manholes to blow their tops?

Manhole explosions are possible side effects of treating roads covered in snow and ice. Crews have responded to at least 300 manhole fires in New York City alone since last week's snowstorm. In Brooklyn, at least two fires broke out within days, reports CBS News correspondent Anna Werner.

The metal manhole covers can weigh as much as 300 pounds. On Monday, an explosion inside a manhole was so powerful it blew the cover right off, injuring a man walking his dog. Another incident just blocks away forced the evacuation of several apartment buildings, and a car was consumed Tuesday morning by a manhole fire beneath it.

Wherever there are manholes, explosions can happen, and winter weather is often to blame.

Al Munro, who supervises repair crews for the electricity provider Con Edison, said the salt spread on icy roads is bad for the 95,000 miles of cable under New York's streets.

"Snowstorm's coming tonight? Mentally, you're just prepared ... for what to expect," Munro said.

Melting snow and salt sparks NYC manhole explosion

When the ice melts, the salty runoff floods into manholes packed with electrical equipment.

If any cable has faulty insulation, the salt can come into contact with the copper wiring, causing a spark that could ignite trapped gases.

"You'll often see in these situations flames shooting out of the hole," Con Edison spokesman Michael Clendenin said.

He said that, five years ago, the utility began installing vented covers that enable trapped gasses to escape, but it's not a perfect solution.

The man walking his dog was struck by a vented manhole cover.

Some might wonder if Con Edison couldn't do more to prevent the accidents.

"We're always looking at new technologies to try to minimize these kinds of events," Clendenin said.

Roger Anderson, who teaches at the Columbia University engineering school, said growing neighborhoods mean more cables underground, which amps up the danger.

"Any stuff you put in increases the risk," Anderson said.

It's like an overstuffed suitcase with seams that are more likely to pop.

This is not just a problem for cold weather cities. Hot weather can also overtax underground electrical systems and potentially start fires or trigger explosions.