IT'S THE most outlandish alien theory yet. Beings from outer space establishing secret bases on Earth, and travelling in ships that are just as much at home beneath the sea as they are in the skies. Beyond belief? Perhaps. But a new book expounding the idea is written by one of the world's most respected authorities on UFOs, Timothy Good, and his findings are supported by one of Britain's most senior military men, retired Admiral of the Fleet Lord Hill-Norton. Here, in the first of two articles, Good sets out his case. Read it, and make up your own mind...

On the morning of September 4, 1971, four mmembers of the National Geographic Institute of Costa Rica were flying in a twin-engined aircraft 10,000ft above Lake Cote, near the Central American state's forbidding Arenal volcano.

A special map-making camera was slung underneath their plane. It was automatic and large-format, and every 20 seconds it took another photograph of the lake beneath.

When the photographs were developed, one of the frames showed what seemed to be a metallic disc about 160ft in diameter, which had just left, or was on the point of entering, the lake. It was giving off light, and had made a sudden manoeuvre at the instant the photo was taken.

The object showed up on neither the previous frame of film nor the one afterwards. Checks on the negative eliminated tricks of the light as an explanation.

What the geographers had seen was an extraordinary but little-known phenomenon - a USO. Unidentified Flying Objects, or UFOs, have been reported for centuries. What few people realise is that USOs-Unidentified Submergible Objects - have been reported for almost as long.

Sometimes, as in the mysterious incident at Lake Cote, a UFO can be seen transforming itself into a USO, or vice versa.

Lake Cote is extremely deep, and there have been numerous other reports of unknown submergible craft entering and leaving it. Local fishermen, out on the lake in the small hours, have seen various objects below them, giving off coloured lights.

Sometimes the movement of these objects has caused the men to lose their balance and risk plunging into the water. They have also been blamed for a reduction in the fish population.

One morning, around 9am, two men heard a metallic noise coming from the lake and saw an object emerge that was shaped like a submarine with three 'domes' on its top. It hung there for a few seconds, then shot off towards the mountains to the north.

Other USOs have been seen in lakes and open seas around the world, sometimes giving out light, sometimes deeply submerged, sometimes just below the surface, sometimes skimming across it - and sometimes, as at Lake Cote, entering or leaving.

Evidently, many of the objects we are accustomed to call UFOs can just as easily travel in water as in the air.

On the night of November 6, 1973, a unique encounter with a USO occurred off the coast of America. Fishermen in Pascagoula, Mississippi, reported it, coastguards confirmed it. There were nine witnesses.

The object was circular or elliptical in shape, about 12ft long, and gave out a beam of amber light. The two fishermen who raised the alarm said it looked like 'a mini-submarine' and hovered about 4ft below them in the water. It was so close that they tried to hit it with their paddles.

HOWEVER, each time they tried to make contact, the light would go out and reappear in another position. When coastguard officers were summoned, they, too, tried to hit the USO. They succeeded, and reported that the object, whatever it was, felt metallic.

The coastguards made detailed notes. The object, they said, had a parachute-like shape and moved at a speed of six to eight knots, making steadily for deeper water.

The intensity of its light varied from nothing to a glare that was sometimes too bright to look at. When a torch was shone at it, the light 'turned off' until the beam was removed. 'The phenomena observed were not consistent with any known fish, other marine life or known light source,' concluded the coastguard report.

So what was in the water- and why was it there?

THERE is one place in the world where reports of Unidentified Submergible Objects are particularly frequent, and particularly strange.

Puerto Rico, the Caribbean's so called Island of Enchantment, lies in one corner of the area that has become known as the Bermuda Triangle, renowned for the disappearances of ships and boats, and for sightings of 'flying saucers'. It is here, in this former US. colony, whose politics and defence are still inextricably bound up with America, that much of my research into the USO phenomenon has focused.

A great deal of the strange activity around the island has centred on the 28,000 acres of mountainous rainforest known as El Yunque, on the north-eastern coast.

Jose Orlando Golis, who works for the Puerto Rican government, lives close to El Yunque. 'Many people have seen UFOs flying over the water close to the surface,' he told me. 'Once, at 1am, we saw one with many coloured lights flying next to the sea just over the surface.

'At first we thought it was a boat. It seemed to be dark underneath and had lights - mostly red and blue at another, upper level. Then it angled and moved upwards. It made a humming sound, and seemed to head in the direction of El Yunque.'

Felix Rivera is a diver with an underwater salvage company based near the American Naval Air Station which adjoins El Yunque, one of many U.S. bases on the island. He confirmed to me that UFOs and USOs have frequently been observed often by American military personnel.

'Navy Seals - America's elite special forces - have seen USOs here,' he revealed. 'Some have told me that these things will often come up close to boats, then shoot off. They move too fast underwater to be ours.'

Another hotspot of activity is off the south-west coast. In an area known as Cayo Margarita, 15 miles out in the Atlantic, many fishermen, as well as commercial and private pilots, claim to have seen USOs entering or emerging from the sea. Fisherman Aristides Medina said: 'Once, I was fishing late at night and two of them passed under my boat, radiating a blue light. On other occasions, I have seen them when they emerge from the water and fly away at great speed.'

He has also seen them plunge into the water - and always in the same area of the sea. It would seem these objects have definite preferences as to which part of the ocean they choose to emerge from and enter.

REMARKABLY vivid sighting was reported by a diver called Inocencia Cataquet. He was in the water off Pena Blancas in the north-east of the island when he came across what appeared to be a disc-shaped submersible craft, lying on the seabed.

The craft seemed to be camouflaged with a sand-like material in order to blend in with its surroundings. Next to it lay a rectangular object, with a transparent cable, which was moving around in the water.

As if in reaction to the divers presence, the cable was suddenly retracted. Unnerved, Cataquet raced for the surface - in time to see the disc emerge from the water with a loud buzzing sound and fly away.

WHAT is going on here? If strange metallic craft are emerging from the waters around Puerto Rico on anything like a regular basis, it is inconceivable that the military authorities could be unaware of it. Some of the American bases in the area have exactly the sort of equipment that would be needed to track an object from the unknown.

For example, there is the huge aerostat, or tethered balloon, that flies 15,OOOft above the island's south-west coast, with radar installations slung beneath it to monitor air traffic. A number of witnesses claim to have seen strange flying objects in the vicinity.

Puerto Rico is also host to the unique radio-telescope at Arecibo, a natural crater turned into the parabolic bowl of a gigantic receiving antenna - the largest and most sensitive radiotelescope in the world.

ONE of the acknowledged roles of Arecibo is as a listening post for alien lifeforms broadcasting from outer space. The project is known as SETI - the Search for Extra-Terrestrial Intelligence.

Yet, ironically, unusual craft and beings have been reported around the radio observatory itself. A large boomerang-shaped object was seen `hanging' above the telescope. One night, witnesses saw a huge disc in the sky, followed by three triangular shaped objects.

Is the paradoxical truth that, while SETI looks for extraterrestrial beings light years away, they are operating on its own doorstep?

On a visit to the site in January 1999, I interviewed the observatory's Dr Jost, Alonso. He told me that many local people believe the observatory is a focus for UFO activity.

`This is a complicated issue,' he said. `Sometimes I think military experiments are responsible for what people are seeing. There are people who say this is a US. government cover-up.

'As to aliens, statistically, there has to be something. There are between 200 to 400 billion stars in our galaxy, and 10 pc of them are of the same type as our sun.

`If other life forms exist, it's only a matter of time and technology before contact is made.'

Certainly, the Arecibo site is a focus for strange phenomena. One of the strangest came in March 1993, when a former British Airways pilot, Graham Sheppard, experienced a mysterious `lateral displacement', throwing him miles off course, as he was flying his Cessna 172 light aircraft close to the telescope.

Sheppard, an expert navigator whose experience in the air spans four decades, and includes flying 747s, describes it as the most bewildering moment of his career.

After passing over the telescope, a feeling of unease and growing confusion came over me,' he says. `Minutes went by with no idea of my position, but I was confident the west coast would soon appear at right angles to my course.

`I recall the shock of seeing the coastline, not at right angles to my course, but parallel. It became alarmingly clear that I was flying along the south coast. The navigation error here is enormous and should be impossible.'

An on-board video camera confirms that Sheppard's gyro compass was set to the correct bearing for his original course. It also confirms that he was flying at 2,200ft - yet somehow he had passed over hills up to 3,900ft high.

THE aircraft's instruments had appeared normal, and later weather checks ruled out a freak wind having blown him off course. None of the pilots to whom I have sent records of this flight can rationalise the displacement.

Sir Mark Thomson, a former Royal Navy Jet pilot who has taken a close interest in the strange events around Puerto Rico, is as baffled as he is impressed.

'It is one of the most important pieces of evidence I've seen,' he told me. `There is no explanation in our laws of physics.'

Could such aerial displacement be linked to the disappearances that have made the whole area of the Bermuda Triangle so notorious - such as the flight of Grumman Avenger torpedo-bombers that vanished off the Florida coast just after World War II?

In Puerto Rico, there are those who report incidents that are equally sinister and inexplicable. I interviewed two groups of witnesses who, on two separate occasions in 1988, claim to have watched as U.S. Navy Grumman F-14 jets were apparently `captured or otherwise absorbed' mid-flight by large, unknown aerial craft they were pursuing.

Perhaps it sounds like fantasy. But on one occasion, at least, it seems that the object which had caused such a disappearance may even have been captured on U.S. radar.

`MAYDAY, Mayday. We can see a strange object in our course. We are lost. Mayday. Mayday'

The distress-call came at 8pm local time on June 28, 1980. Jose Luis Maldonado Torres, a 31-year old pilot, was flying an Ercoupe 415-D aeroplane, together with a 22-year-old student pilot, from Santo Domingo to San Juan, Puerto Rico's main airport.

I have listened to a tape of some of the Mayday transmissions from the Ercoupe, leaked by a Federal Aviation Administration source, and the pilots' anxiety is clear.

`A weird object in our course made us change course about three different times . . . we have something weird in front of us . . . We are right again in the same stuff, sir. . .'

After these words, the pilots were not heard of again. A sea and air search revealed no trace. However, a tantalising clue lies in the official report on the incident.

During the Mayday alert, the American Naval Air Station at Roosevelt Roads in Puerto Rico was in contact with air traffic controllers at San Juan. At 8.16pm, 11 minutes after the Ercoupe's s last communication, the Americans made an intriguing comment: `It looks like we may see a few of them out there...'

What did this cryptic message mean? It suggests that the air station had made several radar contacts in the area where the plane disappeared. One of them, of course, was the Ercoupe. But what were the others? Might one of them have been the `weird object', the disorienting `stuff' that aparently engulfed the doomed plane?

One possibility is that the US. Navy had been monitoring whatever weird phenomenon caused the plane's disappearance.

Jorge Martin, the island's leading UFO investigator, has an important contact within the military, a high-ranking officer connected to the U.S. Navy.

From this source, we learn that the American authorities grew alarmed after a number of military aircraft, as well as private planes, disappeared in an area to the north-west of Puerto Rico known as the San Juan Trench.

A steep undersea cleft in the ocean floor, the lowest point this great chasm lies 30,000ft deep, further below sea level than Mount Everest is above it.

HERE is the deepest point in the Atlantic Ocean. The pressures of sea-water there would crush any surface creature, and any but the most specially-constructed man-made submarines.

According to Martin's source, the Navy and Air Force found that, on a regular basis, a huge disc-shape craft would come out of the sea in the Trench, sometimes 'hanging stationary in the air on a great column of water before vanishing or submerging again'.

If this were not outlandish enough, there is more. Pilots were reportedly ordered to approach the object, only for their jets to 'explode silently, to vanish into thin air'.

Can these astonishing claims be true? And if so, what is the terrible secret lurking beneath the ocean?

A STRETCH of 100 miles of open water divides Puerto Rico from it neighbouring island, the Dominican Republic. That stretch is known as the Mona Passage.

Almost half-way across it, some 45 miles to the west of the last cape of Puerto Rico, lies the small roughly circular, uninhabite Mona Island. At the time of its last communication, the lost Ercoupa aircraft was just to the east of Mona Island.

Other crews crossing the area have reported anomalies affecting their positioning. One pilot in a light aircraft noted an `an uncontrollable spin' of his compass.

Jorge Martin's U.S. Naval office offers a remarkable explanation' His revelations about Mona Island are bizarre, extraordinary, and almost beyond belief.

But when I showed my dossier of evidence was shown to Admiral of the Fleet Lord Hill-Norton, one of Britain's most respected ilitary figures, he had no hesitation in endorsing my findings