After 40 years, federal officials are replacing Alvin, the pioneering research submersible that gave humans an unprecedented view of the ocean depths, with a new, deeper-diving craft that will allow them to explore even more remote mysteries of the deep.

Private and government scientists have used Alvin, a compact, rounded orange-colored craft that looks to some like a bathtub toy, to conduct more than 4,000 dives. The vessel -- which carries two scientists and a pilot -- can probe as far down as 4,000 meters (13,000 feet). Over the years, it helped to locate the wreck of the Titanic, recovered a hydrogen bomb, and discovered hydrothermal vents that spew hot water from the ocean floor and provide a habitat for tube worms and other previously unknown species.

Beloved by schoolchildren fascinated by the sea-floor images it captured, Alvin represented the scientific pioneering spirit of the 1960s, when Americans saw space and ocean exploration as key to the country's position as a world power. But the craft has aged and no longer suits scientists' expanding needs, engineers say, and other countries have developed submersibles that outperform Alvin: Japan has a vessel capable of reaching 6,500 meters (21,320 feet), while Russia and France have 6,000-meter ones. China is building its first deep-diving craft.

To stay in the game, the U.S. government has come up with plans for a $21.6 million replacement, though it still lacks a catchy name. The new craft -- which will be based on Alvin's mother ship, the Atlantis -- will also be able to reach 6,500 meters, allowing it to explore 99 percent of the sea floor.

"The contribution Alvin has made to science is unquestioned," said Robert Gagosian, president of the Woods Hole Oceanographic Institution, which will operate and help fund the new submersible. "Alvin enabled whole new generations of scientists to gain access to a previously unseen world, and for engineers to push the limits of their creativity. The replacement vehicle is designed to continue and extend this legacy."

In addition to being able to dive deeper and faster, the new craft -- scheduled to be launched in 2008 -- will have better visibility and lighting, including five windows with overlapping views, compared with Alvin's three. It will boast a ballast system allowing it to hover at any depth to conduct mid-water research; more space for passengers and their cargo; improved sensors and data collection systems; and higher-speed communication with its surface ship.

"It is a major step forward in ocean exploration and brings a new level of versatility and capability to scientists wishing to pursue research projects on a routine basis in areas they have long wanted to study but have been unable to reach," said Maurice Tivey, the oceanographic institute's acting chief scientist of deep submergence.

With the aid of the new vessel, scientists hope to study how hydrothermal vents, which belch out superheated water, operate on the ridge flanks near ocean trenches and on the deeper portions of the continental margins. Researchers also want to examine the largest daily migration of animals on Earth, in which trillions of organisms rise to the sea's surface at night to feed and then descend to its depths during the day to avoid being eaten by predators.

"We've never had the opportunity to dive down and watch that migration," said Bruce Robison, a deep-sea ecologist and senior scientist at the Monterey Bay Research Aquarium Institute. "We'll be able to watch that like never before."

Scientists are also interested in scrutinizing fluids that flow near Earth's mantle, the layer just beneath the surface crust, and investigating possible new energy sources beneath the sea floor and searching for new organisms.

"There is unquestionably much left to be discovered," said Daniel J. Fornari, senior scientist at Woods Hole.

Hundreds of scientists have spent the past decade discussing a possible successor to Alvin, which is owned by the Navy.

While it has been repeatedly retooled and updated, Alvin has structural drawbacks, including windows that do not overlap, making it somewhat difficult for scientists aboard to direct the pilot toward areas they want to explore further.

Cindy Van Dover, the only woman ever to pilot a deep-sea submersible, said Alvin's system gives the pilots a better view than the scientists, something that would be corrected in the new vehicle.

"The animals are always out front when the pilot's working," said Van Dover, an associate biology professor at the College of William and Mary who occasionally gave up her seat so accompanying scientists could get a better look.

Alvin also has a special buoyancy foam designed to withstand pressure as the sub descends, which costs about $1 million. The foam cracks over time and oil can seep in, requiring expensive repairs, according to Barrie Walden, manager of operational scientific services at Woods Hole.

The new submersible will still have some limitations: It cannot go down 11,000 meters (36,000 feet) to reach the entire ocean floor. The federal government is working on building a remote-operated vehicle that would be able to reach those depths.

To some extent, last Friday's announcement will ease concern in the scientific community that the government would switch to remote operated vehicles only, which are cheaper to build but cost roughly the same to operate. There are a handful of deep-diving ROVs, but scientists say the cameras cannot capture the same breadth of information as the human eye.

"It's like looking with one eye through a pipe," Robison said of remote vehicles. "The human visual system is superior to any camera system."

At the same time they are moving toward a new generation of technology, the scientists who have dived aboard Alvin said they will be sorry to retire the vehicle that served them so well. Walden said he and others will miss a craft that has yet to outlive its usefulness, and Gagosian compared using it to a treasured "professional and emotional relationship." Recalling his first dive in Alvin, he said, "I'll just never forget it. You're going to somewhere where no one's ever gone before."

As for coming up with a name for Alvin's successor, officials say they need more time.

"It's certainly a topic of interest," said James Yoder, ocean sciences division director at the National Science Foundation.