Guant'anamo Bay -- US Navy's 82-year-old Caribbean toehold. This survivor of Cuban revolt and Pentagon budget cutters continues its vigil over Caribbean sea lanes

The marine, helmet on and rifle by his side, peers from the height of Guard Tower 21. ``See that?'' he asks. ``It's a Cuban patrol boat.'' Half a mile away the boat swings sleepily at anchor under an azure West Indies sky. It could be a pleasure cruise in search of langouste, the local version of lobster -- but the two men on deck are wearing battle fatigues, not swimsuits.

Welcome to the US Navy installation at Guant'anamo Bay, Cuba, the base that geopolitics forgot. The oldest US post on foreign soil, it is also the only one in a communist country.

Cuban leader Fidel Castro has never seriously tried to dislodge the Navy here. He does, however, refuse the annual $4,085 payment proffered by the United States for use of the base. ``The story is we rent the place, but he doesn't cash our checks,'' says Capt. John Condon, base commander.

Guant'anamo, a lovely bay on Cuba's southern haunches, has been a favored spot of sailors for centuries. Christopher Columbus landed here in 1492 in search of gold and fresh water; in the mid-1700s the British West Indies Squadron occasionally used the harbor as a base. The US arrived during the Spanish-American War, when marines landed at Guant'anamo for a campaign in the nearby hills. After the war, in 1903, a treaty gave the US rights to the base indefinitely.

In 1956, Castro launched his revolution from the Sierra Maestra mountains just west of here. Occasionally the rebels would harass the nearby base. At one point Castro's brother kidnapped 29 US servicemen returning from leave, but eventually released them unharmed.

On the January day in 1959 when Castro triumphantly raised his flag in Havana, the naval base gates were closed. Ever since, US and Cuban riflemen have eyed one another across a demilitarized zone of mine fields and fences, as unlikely neighbors as history may produce.

``They'll do weird stuff, call you names, throw rocks,'' says Marine Sgt. Mark Floresca of his Cuban counterparts. ``But it's not too bad.''

The most visible victim of the standoff here is the base golf course, once one of the finest in the Caribbean. In the early '60s Castro cut off the base's water. The Navy built a desalinization plant in response, but the water the plant produces is so expensive only the course greens and tees are now watered. ``The fairways get pretty brown,'' says one base officer wistfully.

From the top of a hill at its center, the base looks like any military post with 6,500 residents, one stoplight, four outdoor movie theaters, and a McDonald's under construction. As a fighter plane makes sporadic runs at the target range, its machine guns rasping, frigates glide in and out of the emerald bay.

During the Carter era, some high Pentagon officials judged Guant'anamo an anachronism and considered abandoning it. But the base's utility as a training center for the Atlantic Fleet makes it worth keeping, claim officers here. The harbor is uncluttered, and deep water begins just offshore, so ships can be on station in minutes. Last year, 83 vessels came through for maneuvers.

One of the last US outposts in the Caribbean, the base might prove vital if conflict threatened nearby shipping lanes, says the Navy.

If Cuba were a party to that conflict, Guant'anamo would likely spend much energy just fighting for itself. Marine guards admit that the base's main airstrip, for instance, is within easy rocket and mortar range of the surrounding Cuban-held hills.

The large radar on one side of Guant'anamo is probably an unstated reason the US wants to keep the base. Asked its purpose, officers here refuse to say, and then change the subject. A toehold in a foreign land, Guant'anamo would be an ideal spot for gathering electronic intelligence, such as monitoring Cuban air traffic and radio signals.

For the Navy, there is also perhaps emotional satisfaction in retaining Guant'anamo: What better way for the US military to thumb its nose at Castro? ``This is a highly visible reminder of our resolve,'' says Captain Condon.

The place this resolve would be put to the test is the Guant'anamo fence, a 17-mile perimeter where Marine defenders face a Cuban brigade. In a dusty ride along a stretch of this frontier, Cuban soldiers are not much in evidence. Their guard towers top a few far hills. Bunkers with slit windows are scattered in no-man's land. A military truck stops on a ridge a mile away; the patrol boat sits still in upper Guant'anamo Bay.

In late July, the Cuban premier visited Guant'anamo City, 10 miles to the north, to celebrate the anniversary of the revolution. The Cuban brigade was more active then, and the Marines double-manned their posts, for precaution's sake. But for the most part the standoff here is no longer very tense, according to the Marines. The most serious incidents occur when Cubans sneak up to the fence at night, and toss rocks on the tin roof of the Marine forward barracks.

``We're safe,'' says Col. Sam Adams, commander of the Marine force. ``My Mom worries all the time. I keep telling her, `Hey Mom, it's all right.' ''

Still, the outer regions of the base are strewn with obstacles to impede any Cuban incursion. ``Anchor Valley'' is a gulch filled with old anchors to slow down tanks; another valley is studded with rusty buoys. The only live mine fields maintained by the US military are here, carefully marked on all sides by red signs in English and Spanish. They are a great danger to the local wildlife.

The Cubans on the other side of the fence have mine fields of their own, put in after the 1982 US invasion of Grenada. They are not so well marked. The Cuban soldiers, say US officers, are here not so much to keep Americans out of their country as to keep other Cubans in. Commanders refuse to say how often Cubans seeking asylum jump the Guant'anamo fence. They say only that such events occur frequently. Not everyone who tries succeeds. ``You see lights, you hear shots, you hear screams,'' says Colonel A dams.

About 65 Cuban nationals do cross the fence every day at its northeast gate, without incident. They are commuters, coming from nearby towns to jobs on the base. The remnants of a once-large local work force, all held their positions before US-Cuban relations soured in 1959.

When they arrive each morning, Marine guards escort them in and exchange their Cuban identity cards for American ones. When they leave each night, they pass through a Cuban search station on the far side of a ridge, where they are made to change clothes so they cannot smuggle out anything salable.

The commuters say their daily journey from communism to capitalism and back is now routine. Disparities between the two worlds remain great; food is plentiful on base (it will be even more plentiful when the McDonalds opens in Feburary), but it is still rationed in Cuba.

Rasman Henry Cook is 61, a local pipe fitter. ``It's been a long time since this gate's been closed,'' he says, looking across at a faded sign that announces entry into the Republica de Cuba.

Will the gate ever open again? ``I don't know. I sure wish and hope that would happen,'' he says.