Curious about those curbside nitrogen tanks propped up certain city sidewalks? Not too long ago we got an email inquiring about them—it read: "the random nitrogen tanks everywhere at the corner of city blocks are always bubbling and oozing. Are they dangerous?"

A number of theories have been presented about these things—some of our favorites: they're there to asphyxiate rats, to reduce odors, or to scare off tourists. But most tend to believe Con Ed uses the liquid nitrogen in the maintenance of cables and fiber bundles. However, when we called Con Ed they claimed they have absolutely nothing to do with the tanks, and they pointed their liquid nitrogen-free finger towards Verizon.

Apparently Verizon uses the tanks to pump nitrogen gas into the phone lines to keep them dry. We contacted Verizon to put an end to this mystery once and for all, but the gentleman manning the media relations phone was uninformed on the matter. We'll update this post once someone in the know returns our call.

For now, one online source did get the following explanation from a Verizon employee: "I asked a Verizon technician about these once, and he said that they pressurize underground cables to drive out moisture. The cables are not perfect, and will occasionally leak and allow moisture in that shorts the copper strands. Remember that the cables are often below the water table, and run under creeks, drainage ditches, etc. The nitrogen is used to dry them back out."

In 2006 there was a huge settlement upstate which was the result of a liquid nitrogen tank rupturing, causing the man to lose an arm and both legs. In that case he was delivering the tank, but just imagine if a car or truck crashed into one of these on the sidewalk! Whoever is manning the tanks, it seems a bit dangerous to have them lining the busy streets of New York (a city that doesn't exactly attract the best drivers). And by the way, that warning label reads: "rapid suffocation" and "severe frostbite," amongst other things.

UPDATE: We just spoke with Verizon and they confirmed that some of the nitrogen tanks are theirs, and are there for the above mentioned reasons. They also noted that they are completely safe, saying "if a tank was punctured, nitrogen would dissipate in the air without harming anyone around it, or the environment."