SAN DIEGO - As Republicans partied late into the night at the 1996 GOP convention here, a half-dozen sinister shapes cruised through the black waters of adjacent San Diego Bay, their dorsal fins slicing through the water, leaving swirling eddies as they silently dove to probe the darkness for intruders.

The party-goers were unaware that dolphins - those playful, ever-smiling mammals - were among the tight security measures provided for the convention. These particular dolphins were trained and operated by the U.S. Navy for missions that include detecting swimmers who could be terrorists.

The Navy has worked with dolphins and other marine mammals for more than three decades, until recently in secret. It has trained dolphins to find and mark sea mines and to help recover practice bombs and instrument packages jettisoned from test missiles.

The Navy has struggled to perfect technology to replace its dolphins, spending millions on advanced sonars, clunky submersible robots, video cameras on cables and other devices. But nothing worked as well as dolphins.

In a rare triumph of nature over technology, the Navy has torn up its order of two years ago to get rid of the dolphins. Money is flowing not just to maintain the Navy's 75 dolphins and 19 other marine mammals, but to fund research to expand its understanding of them.

"There are some things that marine mammals can just do better than man," said Jeffrey Haun, the trainer who heads the Navy's marine-mammals program here.

The Navy's renewed interest in dolphins springs in part from its growing concern about sea mines, particularly the "smart" mines that can be sown in shallow, murky coastal waters where the Navy and Marines increasingly operate.

In the past, Navy divers worked from boats to clear these mines. But where U.S. troops may have to fight their way ashore, mine-clearance crews would get picked off by sharpshooters on the beach. Dolphins can find and mark these mines underwater - without being detected.

The dolphins train and work in open ocean, without a tether or harness.

Why don't they swim away to "freedom" as some animal-rights activists insist they should?

Because they are fed well and regularly, get better medical care than most humans, and seem to enjoy working with the Navy.

The dolphins do not place mines or blow them up; they don't operate weapons or kill.

Dolphins that detect unauthorized swimmers are trained to report back to their handler and signal with squeaks that they want a special probe device.

If the handler decides the situation merits it, he will affix the probe to the dolphin's snout. The dolphin will locate the swimmer and jab him with the blunt-nosed probe, releasing a float with a bright strobe light that will mark the swimmer's location.