Benjamin Franklin studied the Gulf Stream, and pirates, smugglers and whalers kept an eye on it. But it took satellites and computers to realize the current's role in weather, fishing and even submarine warfare.

The sweeping swirl of the Gulf Stream is 50 to 100 miles wide and less than a mile deep. It's born in the Gulf of Mexico and shinnies up the East Coast to Cape Hatteras, then shifts eastward to the Grand Banks and Newfoundland, swings over to Europe and eventually returns those chilly waters to the Sargasso Sea.On its way, the Gulf Stream - caused by the interplay between the Earth's rotation and winds driving the ocean's survace - moves not only warm water, but nutrients. This is important, at least as far as fish are concerned, since it makes the stream hospitable enough for fish to spend their lives in it.

"The Gulf Stream is really clear and really dark and certain fish really like that," said Mitchell Roffer, who began studying the current 10 years ago for the National Aeronautics and Space Administration and now uses Soviet and U.S. satellites in advising fishermen from Puerto Rico to Nova Scotia.

Many fish like to congregate at the Gulf Stream's edge, where coastal waters run up along side and get mixed around, said Roffer, whose Miami-based service tells fishermen where these congregants are found on a given day.

The massive current was first identified in the 1500s by Juan Ponce deLeon, who discovered Florida. In the days when sea lanes were the only way across the Atlantic, ships destroyed by storms drifted with the stream. Navigators drafted charts noting the drifting wrecks. They didn't know it, but they were charting the route of the Gulf Stream current that swept the wrecks along. But it wasn't until the 18th century that the Gulf Stream made its way onto a map.

The most famous effort to identify the Gulf Stream was undertaken in the 1770s by Franklin, then a shipping agent in Philadelphia, with the help of his cousin Timothy Folger, a whaler from Nantucket. Franklin's interest was sparked by complaints from colonists that mail took a couple weeks longer to come from Europe than it did when sent east, riding the stream part of the way.

Computers and satellites are finally unlocking the secrets of the current.

"It's the first part of the ocean we actually are forecasting," said Scott Glenn, an oceanographer in Harvard University's Department of Earth and Planetary Sciences who helped Allan Robinson develop the Harvard Gulfcast, the first ocean forecasting system to predict the stream's daily variations.

"People have been making models of the Gulf Stream, but this is the first time they have been applied to forecasting," Glenn said.

Financed by the Navy for use with submarines, the Gulfcast is also of interest to shipping companies looking for a fast ride, meteorologists trying to get a grip on the atmosphere and even pleasure boaters. The Navy's specific interest is in the warm fronts where temperature differentials play havoc with the underwater acoustics used by submarine detectors.

William MacLeish, former editor of Oceanus magazine and author of the book, "The Gulf Stream: Encounters with the Blue God," said the current "makes it possible for you and me to have a fairly decent existence."

"The Gulf Stream is part of the circulation of the whole ocean," said oceanographer Carl Wunsch of the Massachusetts Institute of Technology. "It ... transports heat from near the equator toward the polar regions. It makes the mid-latitudes of the Earth habitable.

"We have a much milder winter than they have in Kansas because we're close to a water system that stores a lot of heat."

Those who study global warming are checking out the Gulf Stream, too. One theory holds that if subpolar warming becomes too great, the stream will weaken because a smaller difference in temperature lowers the velocity of the stream's engine. Europe would chill out as a result. Another theory predicts the warming will expand the already warm Sargasso Sea. Westerlies would move that warmed air to Europe and turn the continent into a steam bath.

"Take your choice," MacLeish said. "We are living in great peril."