What lies beneath?

Roger Stokey throttles back on the twin Evinrudes that have sent the pontoon boat and its four occupants skimming out of Woods Hole harbor. The bow gently settles into the water, allowing Greg Packard to take a seat in front of a small TV monitor.

"Is this the only channel we get?" Mr. Packard asks with mock dismay.

"It's all Remus, all day," Mr. Stokey replies.

Remus, a torpedo-shaped underwater vehicle, is out on maneuvers this afternoon. The researchers at Woods Hole Oceanographic Institution (WHOI) here on Cape Cod are trying to get this mechanical mini-Shamu to glide through an underwater hoop, using sonar. Their ultimate goal: a self-docking robotic explorer.

The trial represents one small step toward a new approach to teasing secrets out of the briny deep. Scientists worldwide are laying plans to knit together existing observatories and build new ones in an unprecedented effort to uncover the intricate facets of one of the solar system's least understood realms - Earth's oceans.

Using the latest in remote sensing, power delivery, robotics, and telecommunications technologies, marine scientists are pushing to make long-term measurements of everything from water chemistry, currents, and sea-floor hydrothermal activity to the rise and fall of plankton populations.

The potential payoffs are significant: much improved weather and climate forecasts, more effective ways to manage coastal ecosystems, sustainable fisheries, and volcano and earthquake data that could mitigate related disasters.

Even space exploration could benefit. Jupiter's moon Europa is thought to host an ocean beneath a surface layer of ice. Marine observatories on Earth could act as test beds for the robotic vehicles and suites of sensors needed to study Europa's ocean and search for possible hydrothermal activity and, perhaps, life.

By the standards of "big" science, the price is small: a few billion dollars. That would amount to a small fraction of the cost of the International Space Station or a new high-energy particle accelerator.

Momentum for a global network of ocean observatories has been building for years, as the number of disparate ocean observing systems have grown. But 2003 appears to mark a turning point.

In February, members of the European Union agreed to develop a sea-floor observatory network that could stretch from the Arctic through the Strait of Gibraltar to the Black Sea. The number and locations of the observatories are still under discussion.

Meanwhile, the United States is moving forward with two major federally funded marine-observatory initiatives.

Their architects expect the efforts to yield breakthroughs in basic ocean science as well as in techniques for forecasting a range of marine conditions - from the effects of cold water welling up on coastlines to harmful algae blooms and beach erosion.

The growing interest in using fixed instruments to look at the same patch of ocean for a long period of time reflects a sea change in the questions marine scientists are trying to answer, according to Robert Detrick, a marine geophysicist at WHOI.

For years, ocean expeditions tended to focus on measuring conditions or features at the handful of places that researchers found interesting.

"Now, people are pushing to understand change in the ocean" and its interplay with changes in climate and marine ecosystems, Dr. Detrick says.

Relying solely on ship expeditions also can rule out the chance to study some of these changes at their most interesting turning points, adds colleague Margaret Tivey.

"The ship has to go during certain weather windows. The ship can't go when the weather's poor. Now, if you're trying to find out about beach erosion or shelf erosion, when do you think [these] things happen?" she asks.

With observatory sensors on the sea floor and along buoy mooring lines, however, it's possible to watch foul-weather processes unfold as they happen from the safety of a warm, dry office.

Yet for all the growing interest in ocean observatories worldwide and an ever- increasing emphasis on international cooperation, the US programs still represent what many agree are the most ambitious approaches yet proposed.

As part of a broad international observing effort, the National Oceanic and Atmospheric Administration (NOAA) aims to establish a set of coastal observing systems in the US that would monitor near-shore conditions on unprecedented scale. They would draw on data gathered by a range of private, government, and university installations.

The program could cost $500 million and would bring the government's total investment in what he terms "operational" ocean observatories to between $1.25 billion and $1.5 billion. Operational observatories, as opposed to research sites, would provide steady streams of data to everyone from weather forecasters to shippers.

And that could lead to billions of dollars in additional economic activity, says Richard Spinrad, associate administrator for NOAA's National Ocean Service. For example: Conditions such as temperature and salinity affect water's density and thus a ship's buoyancy.

"We're talking about huge vessels," Dr. Spinrad says. So if water conditions cause a ship carrying cars to sink 1 inch too close to the bottom of a harbor, that could mean $3 million worth of cars unable to be brought into port. Better forecasts could head off such problems.

One example of such an operational observatory is the Gulf of Maine Ocean Observing System, dubbed GOMOOS, Spinrad says. Set up as a prototype, GOMOOS is beginning its third year of operation, according to Neal Pettigrew, a University of Maine oceanographer and chief scientist for GOMOOS.

Dr. Pettigrew and his colleagues have worked to pioneer buoy designs, computer-modeling techniques, and protocols and standards for data and instruments that could be incorporated into a national ocean-observing system.

The system combines high-tech buoys and other instruments with coastal radar capable of tracking waves and ocean currents out to 120 miles offshore.

Pettigrew says the user base for the information GOMOOS gathers is growing and includes Coast Guard search-and- rescue teams, harbor pilots, fishermen, and even the National Weather Service, which taps meteorological data as well as visibility and wave information that the GOMOOS buoys gather.

Preliminary estimates of the benefits from the observatory peg its value at $30 million, compared with a $6 million investment to establish GOMOOS, according to a study by researchers at WHOI's Center for Marine Policy.

"A five-to-one return? That sounds like a pretty good investment to me," says NOAA's Spinrad. "If that same proportion holds nationally, we'd reap a $10 billion gain for our $2 billion investment."

Even as NOAA is taking its coastal-observatory concept out for a test drive, the National Science Foundation (NSF) is preparing a more ambitious program. It aims to establish coastal, regional, and deep-ocean observatories that can help answer fundamental oceanographic questions while developing the cutting-edge sensors and robotic explorers that eventually could work their way into observing systems designed for broader consumption.

The scope of the project is unprecedented. "I don't think anything's been tried like this before," says James Yoder, director of the ocean-sciences division of the NSF. The estimated cost of the agency's Ocean Observation Initiative sits at $200 million just to lay cable, put out buoys, and provide power.

In January, marine scientists are scheduled to gather in San Juan, Puerto Rico, to come up with their short list of critical instruments to connect to the ocean initiative's backbone.

The element that pushes the technological envelope to its ultimate is Neptune, a US-Canadian observatory that would set up an array of buoys, sensors, and "smart" autonomous underwater vehicles that would cover an entire crustal plate and the waters above it.

Satellite links would relay instructions to the AUVs - perhaps docked at stations along the buoys' mooring cables. Once the AUVs returned, they would dock, offload their data, recharge their batteries, and stand by for another mission.

Much of the development work is taking place at the Monterey Bay Aquarium Research Institute in Monterey, Calif.

The effort also has piqued the interest of NASA's Jet Propulsion Laboratory in Pasadena, Calif. JPL's interest in Neptune can be traced to the lab's longstanding effort to study its home planet, notes Patricia Beauchamp, who heads JPL's Neptune-project office. But the R&D effort also has implications for future missions to Europa.

"We're very interested in trying to figure out how to work in the oceans," she says.

Back on the powerboat off Woods Hole, Stokey, Mr. Packard, and research assistant Amy Kukulya are puzzling over how to get Remus to work in the oceans as well. It scoots around the boat just beneath the surface as it lines itself up for the final attempt to nose through the target.

The first attempt succeeded. The next eight failed, although Packard says he suspects that Remus at least hit the sides of the target a couple of times. Now comes try No. 10, and Packard groans. "Missed! Well, thanks for playing."

The team hauls Remus out of the water, hoists it back on its rack at the stern, and heads home.

As if to soothe dashed hopes, Stokey reminds a visitor, "This was just an experiment."

The oceans contain:

• 80 percent of all life on Earth, most of which remains undiscovered.

• 97 percent of the Earth's water.

• Enough salt to cover the Earth's land surface with a layer more than 500 feet thick, roughly the height of a 40-story office building.

• The first plants on earth, the algae, developed 3.5 billion years ago. [Editor's note: The original version gave the incorrect age of algae.]

• The equivalent of a new Exxon Valdez oil spill every eight months, due to oil running off streets and driveways.