Media playback is unsupported on your device Media caption Alvin project manager Kurt Uetz and pilot Mike Skowronski give a sneak peek of the upgrade's results

One of the giants of ocean science is about to get back in the water.

Alvin, the famous US manned deep-submersible, is nearing the end of a major $41m refit.

It will go on a series of engineering test dives in the coming weeks before resuming research duties in May.

This remarkable vessel has notched up many firsts during its 49 years of service, not least the discovery of volcanic vents on the Pacific Ocean floor in 1977.

Before Alvin's crew saw the vents' extraordinary array of animals thriving in the mineral-rich, hot waters gushing up through cracks in the rock, everyone assumed all the deepest places in the oceans would be like deserts - there would be little or no life.

The sub's observations transformed ideas about where and how life could exist, and not just on Earth.

Alvin is currently sitting in a workshop at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts.

Engineers are on a tight deadline to get the upgrades ready for sea trials that will be conducted off Bermuda in April.

Assuming the US Navy certifies all her modifications as fit and ready, the submersible will then be despatched to the Caribbean to resume its science by making a number of descents to the Cayman Trough.

"The work we've been doing on her this past couple of years will give her a whole new lease of life," Kurt Uetz, the project manager for the sub's upgrade, told BBC News.

The researchers who ride to the bottom in the "new" Alvin should notice a huge difference in the vehicle's capabilities, and especially the conditions in which they themselves have to work.

A key upgrade in the National Science Foundation-funded work has been the installation of a new $10m titanium pressure sphere.

This is the "cockpit" in which the pilot and two scientists sit for the duration of a dive.

Weighing five tonnes, this protective ball is only 16.2cm (6.4in) larger in diameter than Alvin's old sphere, but the visibility it now offers to its crew is greatly improved.

Image copyright Other

"The previous sphere had three viewports - one for each crewmember. The difficulty was that those viewports looked in completely different directions from each other. So you could never see what the other people were seeing," said Susan Humprhis, a WHOI senior scientist.

"The new sphere has five viewports, three of them forward-looking, all with overlapping fields of view. The visibility is going to improve immeasurably."

British acrylic manufacturers have supplied the thick conical windows.

Image copyright Other Image caption Alvin regularly undergoes a service but this latest upgrade should transform its capabilities

Other improvements include a new flotation foam, a new command-and-control system, better lighting and hi-def cameras, increased data-logging capabilities, and better interfaces with the science instruments.

Alvin will also have a greater carrying capacity, doubling the permitted payload to about 180kg (400lb). This load might be experiments taken down to the ocean floor or samples that are brought back up.

Many of the changes to the sub, like its new pressure sphere, are there to enable the vessel to go deeper than its previous operational limit of 4,500m.

The plan is to permit the sub to dive to 6,500m, giving it access to 98% of the ocean floor. Only some really deep trenches would remain out of reach.

Hydrothermal vent systems

Their existence was first established in 1977 off the Galapagos Islands

Since been seen at many volcanic sites, down to 5,000m at Mid-Cayman Rise

Water drawn through sea floor cracks is heated and ejected via vent openings

Hot fluid carries dissolved metals and other chemicals from under ocean floor

Vent systems support an extraordinary array of microbial and animal lifeforms

Beyond the reach of sunlight, ecosystems depend on chemosynthesis

But this objective is still a little way off. Some elements of the vessel, such as its thrusters and the variable ballast system, have yet to get their overhaul; and until they do, Alvin will not be certified to descend to the new depth.

"One thing we'd need is a better energy source," explained Mr Uetz.

"Currently, if we go to 4,500m, it's an eight- to 10-hour dive, but to go to 6,500m we're going to need at least 12 hours. So right now we're looking to move from lead-acid to lithium-ion batteries and we're working with the US Navy on a test programme that would allow us to do that."

Dr Humphris is impatient. "It's going to be very exciting," she said.

"You know, we've see less than 1% of the ocean floor and we still know so little about how ocean processes work. I think we're going to learn a lot more about what the ocean floor really looks like. And the other places we'll be able to go will be the shallower parts of deep-ocean trenches, and those are very active places where there are a lot of earthquakes and which are also associated with volcanoes."

The French, the Russians, the Japanese and even the Chinese now have manned subs that can go deeper than Alvin's current limit. People will also recall movie director James Cameron visiting the deepest part of the ocean in a vehicle he himself had commissioned.

Media playback is unsupported on your device Media caption Susan Humphris: The upgrade will change the way we do science on the ocean floor

What none of those vessels can match, of course, is the WHOI vehicle's heritage: a total 4,664 dives over nearly five decades.

One thing that will not change in the future is the rule that governs who gets to choose the music played inside the sphere on the long descents and ascents.

"Music is at the pilot's discretion," said Mike Skowronski, one of the select group of individuals who gets to take the controls of the famous sub.

"Scientists aren't allowed to bring their own music in the vehicle. It's more for the pilot's comfort and keeping him in a certain rhythm and to help him maintain his concentration."

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos