AT&T has put down $6 million for a one-of-a-kind piece of construction gear: a robotic tractor that will crawl across the ocean floor almost a mile down, cut through rock and mud in the underwater gloom and cover the company's precious communications cables.

American Telephone & Telegraph Co. is busy stringing a new generation of high-capacity cables employing optical fiber across oceans -- 18,690 miles of them to date. Complicating the job is the fact that the cables have numerous natural enemies.

An AT&T cable off the coast of South Korea was recently damaged by fishermen driving poles into the ocean floor. Bottom-dragging nets occasionally snag on cables and cut them. "Fishermen are going deeper and deeper," AT&T engineer John Maclay noted. He said the company therefore is being forced to protect the cables in deeper waters.

Sharks are attracted by the electrical emissions of the cables and are known to gnaw on them. And at the shore line rough surf can savage exposed cables, which transmit calls as blips of light.

AT&T won't say just how often the cables are cut, considering it a company secret. But the company is working to reduce the frequency by burying the cables, a job now done tediously by a variety of means -- with backhoes on the beach, divers from rafts and small boats in shallow water and less-advanced forms of sub-sea contraptions in deep water.

The tractor is to be built by the British firm Soil Machine Dynamics and enter service in about a year. It is intended to begin the coverup on the beach, roll out through the surf and keep on going until the cable reaches a depth of 4,550 feet, at which point the danger of disruption has significantly decreased.

Using jets of high-pressure water and a rock-cutting buzz saw, it will dig a yard-deep trench as it goes, lay the cable and cover it, all at a rate of 3 feet to 15 feet per minute. Cameras and floodlights will enable engineers on the water's surface to steer the 10-ton machine around ocean-floor obstacles. A manipulator arm will handle delicate jobs.

AT&T frequently vies for cable-laying jobs with competition from British and Japanese firms, and it hopes the tractor will help it win more often. "We should be able to get a more difficult job done more quickly," Maclay said.