Kaiko, which entered the record books in 1995 by diving 36,008 feet to the bottom of the Challenger Deep — the ocean's deepest point — snapped its tether as a typhoon approached in late May and has been missing since then.
Barbara Moore, director of the undersea research program at the National Oceanic and Atmospheric Administration in Washington, called the loss "very disappointing."
"It was a one-of-a-kind instrument," Moore said. "It's had a good and productive career; nevertheless, it still had a lot of life left in it. It's a real loss to the scientific community."
The Japan Marine Science and Technology Center, which runs the bright yellow, 10-foot-long unmanned submersible, will decide Thursday whether to continue searching for it, spokesman Tomoaki Kanai said.
"We have no idea why it broke free. This is a first. But if we lose it, it's going to have a big impact on deep-sea research," Kanai said.
Equipped with two robot arms and four television cameras, the $15 million Kaiko is the world's only probe that can go deeper than 4.34 miles.
The submarine has taken samples of new bacteria that Japanese researchers are using to develop new medicine. It has also studied shifts in deep sea crusts and analyzed deep sea life forms that may hold insights into life on other planets.
Among its discoveries was the existence of 180 kinds of micro-organisms in sea mud at depths of more than 6.25 miles, where the water pressure is 1,000 times stronger than air pressure at sea level.
As a salvage vehicle, Kaiko helped locate a Japanese rocket that went down over the Pacific and helped recover a Japanese fishing ship that was accidentally rammed and sunk by a U.S. Navy submarine in 2001 off Hawaii.
On May 29, Kaiko was conducting earthquake research on the sea floor some 2.9 miles below the surface, off southern Japan, when a typhoon approached. Operators on the mother ship decided to reel in the probe before the storm struck and discovered that the 5.6-ton Kaiko had broken free, Kanai said.
Kaiko is designed to float to the surface and emit a tracking signal if its tether is broken. Although searchers briefly detected the beacon, they were unable to locate the probe and suspected it has either drifted off site or sunk to the bottom.
Among the world's other deep diving submersibles are the center's own Shinkai 6500, which can carry three researchers to a depth of 4.1 miles; France's Nautile, which holds a crew of three and can dive to about 3.75 miles; and Russia's unmanned MIR vehicles, also capable of reaching that depth.
The deepest point ever reached by a manned probe was 35,810 feet by the U.S. Navy's Trieste 1 in 1960, at a site about 25 miles away from the Challenger Deep in the Pacific's Mariana Trench. The Trieste 1 has since been decommissioned.
Japan Marine Science and Technology Center has not decided whether to build a replacement if Kaiko cannot be recovered.