''It took several years for scientists to consider the possibilities,'' said Dr. Alan Chave, a senior scientist of the Woods Hole Oceanographic Institution in Massachusetts. ''But last year it all came together and we showed that continuous deep-sea seismic sensing was possible, using these old cables.''

The world's first deep undersea seismic observatory capable of continuous long-term functioning began operating last September. Its seismometer failed two months later because of a short circuit, but Dr. Chave and his colleagues plan to retrieve and repair the instrument in September. With several upgrades now completed, the observatory (called ''H2O,'' standing for ''Hawaii-2 Observatory'') will then resume operation, midway between California and Hawaii, at a depth of 16,400 feet.

A feature of the unmanned sea-floor observatory is a junction box equipped with eight power outlets and signal connectors allowing scientists to plug more ocean-bottom sensors into the line. Among the supplementary instruments scientists plan to install is a hydrophone capable of listening to whale calls and tracking their migrations.

The cost of H2O, financed by the National Science Foundation, was about $2.5 million. If the project had had to start from scratch by laying its own cable, it would have cost up to about $120 million, scientists estimate.

The idea that led to the project dates from a decade ago. In 1988 a scientist at Tokyo University suggested that abandoned telephone cables might be reused for research, and that suggestion started American scientists thinking. Eventually, a consortium that included Woods Hole, the University of Hawaii and IRIS came up with a plan. A shore-based power station could pump direct current at 5,000 volts into one end of AT&T's broken cable, creating a thousand-mile-long extension cord to power scientific instruments three miles deep. Electricity flowing into the cable would move along it, powering sensors and repeaters, and finally grounding the current into the ocean at the severed end, thereby completing a circuit.

But making H2O a reality was a hair-raising challenge, as Dr. Chave described it.

The tools included the 270-foot research ship Thomas Thompson; the Jason, a deep-sea remotely operated vehicle; and the Media, a remotely operated camera platform to watch the Jason from above, helping Jason's pilot aboard the Thompson to avoid entangling obstacles three miles below him.

The scientists focused their efforts on the part of the cable reaching from dry land in Hawaii to a spot 1,000 miles to the east -- a smooth region of sea floor where they expected seismic signals to be especially useful to geophysicists. Searching the ocean bottom for the cable last summer, the team found it nearly one mile from where they had expected.