It is one of the most abundant substances on Earth, was once used as currency and costs politicians their jobs if there isn't enough.
Salt, the ubiquitous substance that helps clear our highways of snow, is mined in a half-dozen states from deposits formed when the oceans that covered the continent receded 500 million years ago, leaving behind the salt from the water.
Used by the Romans to pay soldiers, the compound known as sodium chloride has helped clear snow from the roads in New England since the 1940s and has been a necessary weapon in the nation's war on winter for 40 years.
Despite the damage it causes to concrete, pavement, sidewalks and waterways, we use 16 million tons of it on highways nationwide each year.
Each fall, the state of Maryland stocks 230,000 tons of salt - enough to fill about 30,000 dump trucks - transporting it by truck from ships unloaded at the port of Baltimore to salt domes and cavernous storage barns across the state.
It is a chemical people hate on their cars, but demand on their roads. And shortages can be politically devastating.
The late Michael A. Bilandic was defeated in his bid to be re-elected as mayor of Chicago in 1979 shortly after he failed to sufficiently salt the city's roads during a blizzard. Shortages of salt for Baltimore County's roads in early 1994 were largely seen as contributing to the defeat of County Executive Roger B. Hayden later that year.
There are alternatives for melting snow and ice. But salt - specifically sodium chloride - is the most widely used for three reasons: chemistry, cost and climate.
Most snow in the United States falls when the temperatures are between 25 and 32 degrees Fahrenheit, which makes salt an ideal compound for winter because it works until temperatures drop to minus 6 degrees, a level at which salt itself will freeze.
In places such as Alaska, Canada, and the Upper Midwest, where temperatures frequently dip to subzero levels, highway crews often lay off the use of salt altogether because the weather is too cold for it to work.
Crews in such frigid areas repeatedly plow to keep the roads as clear as possible of moisture and when a lot of snow accumulates, they sometimes spread sand and crushed stones, which cannot melt snow but will increase traction.
"When you get down to zero degrees, or 10 below zero, the best thing to do is just make sure the roads are as clear as possible," said Bruce Beltram of the Salt Institute, an agency that represents salt wholesalers and dealers.
Salt's effectiveness also depends on the amount of snow on the roads.
Accumulations of more than three inches make the snow too deep for salt to be effective, said Sandra Dobson, a spokeswoman for the State Highway Administration.
As with other major storms, Maryland highway crews stopped salting and focused on plowing during the height of this week's storm because so much snow fell so fast, she said.
But Dobson said that crews were applying salt yesterday to several major arteries in the Baltimore area because enough snow had been cleared to let the salt work.
"We're only salting when we get down to about 2 inches of snow on the road," Dobson said yesterday afternoon.
How salt works is a matter of simple chemistry: Salt enters the snow, absorbing the moisture from it, on contact, the way that a sponge or towel will absorb drops of water.
As the salt crystals mix with the snow and ice, the crystals go into solution and form a brine that has a lower freezing temperature than snow around it. This in turn, melts the snow around the brine, turning the snow to a salty solution that is less treacherous than the salt on the roadways.
The traditional salt used is compound of two chemicals: sodium and chloride, which is the same chemical mix found in table salt.
When temperatures dip down to salt's minus-6 freezing point, highway crews often switch to organic compounds, liquid treatments, such as a salt brine, or alternative forms of salt, which includes calcium chloride, which has a freezing temperature of minus 58 degrees.
Many alternatives are promoted as being more effective at colder temperatures and causing less environmental damage.
Cargill Salt, a Minnesota-based salt producer, has sold an organic compound to the cities of Minneapolis and Rochester - as well as Baltimore - that contains molasses.
"It doesn't bounce, scatter and end up in a ditch," said Michael Hoerle, a Cargill sales representative in Plymouth, Minn.
For the state's chilliest storms in Garrett County, the State Highway Administration uses Ice Ban M-50, an agricultural byproduct that has a freezing point of minus 36 degrees.
Not only are alternative salts and treatments more expensive, but they don't always work.
For years Maryland used magnesium chloride as a pretreatment for snow storms predicted to begin at rush hour. If spread on roads just before an expected storm, it would prevent the snow and ice from bonding with the road surface, Dobson said.
But the State Highway Administration suspended the use of magnesium chloride last week after state officials learned that is created slippery conditions in Montgomery and Prince George's counties.
Dobson said the SHA is investigating the problem.
Those charged with clearing the highways say that salt remains the most effective tool for the job. At an average price of $35 a ton, it also is the cheapest.
"Overrall, it's still the most effective thing we have out there," said Rodney Wynne, research manager for the Maryland State Highway Administration.
But salt has its drawbacks.
When salt mixes with the water from the melted snow, it seeps down into tiny cracks in a paved or concrete surface and expands when it refreezes, creating cracks as it pushes against the concrete or pavement.
When salt corrodes steel, the rust that results occupies four times the volume of the steel it replaces, which can also break up any concrete that is being used to reinforce it.
Bertram and other experts say the key to avoid cracks is to remove the slush formed by the salt and the melting snow as quickly as possible from a sidewalk, roadway or metal surface.
Salt may also have long-term effects on waterways.
Road salt gradually sinks into the ground, just as other chemicals sink in.
"The salt accumulating in our environment - it's not breaking down, it's not disappearing from our waterways," said Richard Forman, a professor of landscape ecology at Harvard University who just completed a book on the effects of salt and other materials on the environment.
Forman said there have been no comprehensive studies on the effects of salt on the environment, but many scientists say it could be affecting fish and algae that live in ponds and streams near heavily salted roadways.
Environment Canada, the nation's equivalent of the U.S. Environmental Protection Agency, said in a December 2001 report that salt was causing enough harm to the ground water, plants and wildlife to warrant a comprehensive plan for its management.
But those in the salt industry say that the price and effectiveness of salt still make it the weapon of choice, even when balanced against environmental concerns.
"There's always been talk about salt's environmental effects, and how it corrodes concrete and metal, but the fact is people need to get to work in the morning, whether or not there's a foot of snow on the ground, and salt is what helps them get there," said Skip Niman, a research chemist, consultant and former director of research at Cargill Salt.