By Virginia Phillips
All great careers come to an end and the deep-sea manned submersible Alvin goes into retirement after 40 years of remarkable work in the world's oceans.
Even with Alvin we know so little about the deep ocean
The sub has taken 12,000 people on over 4,000 dives, to observe the lifeforms that must cope with super-pressures and move about in total darkness.
It is said Alvin research has featured in nearly 2,000 scientific papers.
It helped confirm the theory of plate tectonics and continental drift, and discovered hydrothermal vents.
But Alvin's capabilities are now being stretched - Japan, Russia and France have more advanced vehicles - and it will be replaced. The US National Science Foundation (NSF) recently announced it would fund the new vehicle, which will be operated, as before, by the Woods Hole Oceanographic Institution (WHOI) in Massachusetts.
"I firmly believe that a good instrument can measure almost anything better than a person can, if you know what you want to measure. People are so versatile; they can sense things to be done. I find it difficult to imagine what kind of instrument should have been put on the Beagle instead of Charles Darwin."
So spoke Allyn Vine of WHOI back in 1956 at a symposium on manned undersea vehicles. His philosophy was simple: if you are going to observe something, you cannot do better than send real people to do it.
And it was in that spirit that Woods Hole designed and built Alvin - affectionately borrowing from Allyn Vine's own name.
ALVIN - RESEARCH SUBMERSIBLE
1st human-occupied vehicle (HOV) in routine use
Can reach 4,500m; 63% of ocean floor accessible
Launched from deck of Research Vessel Atlantis
Discovered 'black smokers'; explored RMS Titanic
Alvin, which first went into service in 1964, is capable of plunging to nearly 5,000m - around half the height of Everest. But deep-sea exploration is not without its dangers.
"I've been at sea when one of our instruments has sprung a leak at 3,000m and comes back completely wrecked," Dr Bram Murton, from the Southampton Oceanography Centre, UK, told BBC Radio 4's Material World programme.
Murton has spent many an hour gazing through Alvin's portholes carrying out research.
"Where I was diving was in the Mid-Atlantic - I climbed into this stubby little submersible carrying my woolly hat. To me it was like going into space, a completely different environment.
"The dive takes about 2½ hours to get to the bottom. On the way down the whole thing's creaking and groaning as the pressure builds up. You get near the bottom, put the lights on and this moonscape appears. A truly awesome experience."
It is something which Dr Bob Detrick, a senior scientist at Woods Hole, would love to experience, too. Sadly, he has never dived in Alvin but if he were able to, he would have to travel light - there is not much room on board.
"The pressure sphere is just a little smaller than two metres in diameter, much smaller than the big ballistic attack submarines," Dr Detrick said.
"It has three windows or portholes that two scientific observers and a pilot can look out on. It looks a bit like a praying mantis with a couple of mechanical arms that it can reach out and grab things with.
WHOI REPLACEMENT VEHICLE
Faster descent and forward speed; hover capability
More space; five view ports instead of current three
Heavier science payloads; more sample storage space
Better data links with surface, and via satellite to shore
"And it has various cameras and light systems to illuminate the sea floor because at these depths there's no sunlight."
Alvin's achievements are far-ranging. It rescued a hydrogen bomb from the Mediterranean Sea at just two years old. It was one of the first subs to explore the Mid-Atlantic ridge, and it has even starred in the movies.
But it is none of these which score the highest in Detrick's book.
"I think scientifically the most exciting discovery was made in 1977, when Alvin participated in the discovery of hydrothermal vents in the Galapagos region.
"This very strange world with exotic forms of life that exist in total darkness on the sea floor was completely unexpected. And I think this is classed as one of the great biological discoveries of the 20th Century."
Existence first established 1977
Associated with volcanic activity
Water drawn through sea floor cracks is superheated and ejected through vent openings
Hot fluid carries dissolved metals and other chemicals from beneath ocean floor
Evolution of extraordinary organisms around vents
Chemosynthesis process sustains ecosystems - not photosynthesis
It is a tough act for a replacement to follow. But Detrick is looking forward to the next instalment in this underwater adventure.
"The new vehicle will be bigger; 2.1m in diameter, which doesn't sound like a lot. But in terms of volume, it is 27 cubic ft larger, so it will be much roomier inside," Detrick said.
"And the new Alvin's being designed to dive deeper. The present vehicle can dive to about 4,500m, which allows it to access the sea bed over about 63% of the ocean's floor.
"The new vehicle's designed to dive to 6,500m - about 40% deeper. That will give it access to over 99% of the sea floor."
When the new Alvin comes into service in 2008, it may even have the power to transmit real-time images of the ocean floor to your TV.
And what particular hi-tech request does Bram Murton have for a feature on the new sub? "Well, I think a coffee machine for sure," he says.