By Mark Schrope

Since it was first launched in 1964, the venerable Alvin submersible, owned by the US Navy and operated by the Woods Hole Oceanographic Institution (WHOI) in Massachusetts, has enabled numerous historic discoveries.

On Tuesday, Alvin was plucked from the sea for the last time in its current form. With the sub's final mission complete--it has been studying the depths just a few kilometres from the site of the Deepwater Horizon oil spill in the Gulf of Mexico--it will now undergo a $40-million rebirth as a submersible with dramatically improved capabilities.

Although Alvin has had countless improvements and overhauls, and no original parts remain, it has maintained largely the same form since it was given a new titanium hull in 1974. With more than 4,000 dives under its belt, the submersible has become an iconic part of oceanography. "I suspect that 50 or 100 years from now, Alvin is something people will remember and write about, in that it changed the nature of what we know about the planet," says Bruce Strickrott, the Alvin team's expedition leader.

During its long life, Alvin has been attacked by a swordfish; enabled the discovery of deep-sea hydrothermal vents; took humans to the wreckage of the Titanic for the first time; and helped to recover a lost hydrogen bomb. Between 1968 and 1969 it even sat on the sea floor for 10 months after a launch accident. After its recovery, researchers were surprised to find sandwiches inside that were still in fair shape.

In 2008, the WHOI abandoned plans to entirely replace Alvin after deciding it would be too expensive. They then settled on a scheme to transform the current sub in two phases while balancing cost, engineering possibilities and research priorities.

The first phase, funded largely by the US National Science Foundation (NSF) and likely to stretch into 2012, will essentially involve removing and replacing the entire front half of the sub. Phase two, with details and timing still to be finalized, will transform the back half.

Extreme-depth makeover

The most substantial initial change will be a larger titanium sphere to carry crew. It will be rated for dives up to 6,500 meters, although the sub will retain its current 4,500-metre rating until phase two is complete. Alvin is one of only a handful of submersibles on the planet capable of dives deeper than 1,000 meters.

Today's Alvin has just three 15-centimetre portholes, one facing forward for the pilot and two on the sides for observers, robbing scientists of prime forward views. The new sphere will have three forward-facing 18-centimeter portholes as well as two smaller side windows. And whereas divers previously sat sprawled against the sub's confined walls, everyone will now have adjustable seats to work from.

Other major upgrades will include fiber-optic cables, new cameras and lights, manipulator arms capable of swinging out for forward or side activity, replacement of analogue controls with a digital touch-screen system, and an advanced autopilot system. Phase two upgrades, estimated to cost about $7 million, will focus on improving the sub's battery and ballast systems.

"It's really exciting, our workhorse is getting revitalized and coming back even stronger," says Tim Shank, an evolutionary biologist from the WHOI who participated in the Gulf of Mexico expedition and is a veteran of 55 dives in Alvin.

Oil mission

The retirement of Alvin was postponed to allow an unscheduled NSF-funded expedition to study the effects of the Deepwater Horizon oil spill on the deep sea. Film-maker Mike deGruy and expedition chief scientist Charles Fisher, from Pennsylvania State University in University Park, took the last dive. "It was an honor," said Fisher of his prized slot, just after surfacing. Famed marine biologist and National Geographic explorer-in-residence Sylvia Earle was also present as part of filming for a documentary called Mission Blue.

The dive team was returning to a large area of corals apparently damaged by oil from the spill and first discovered in November using a remotely operated vehicle. Using Alvin, this time they gathered images, sediment and soil samples, and positioned a time-lapse camera that will operate over the next two months. This will create a record of how the corals repair or deteriorate in response to their recent damage, offering clues to long-term impacts. The researchers have been working within the Natural Resource Damage Assessment process, which legally establishes the damages for which BP and others involved in the Deepwater Horizon spill could be held liable.