ALMOST invisible to radar, the F-117 Stealth fighter is one of the most sophisticated warplanes ever built.

But for seven years the US Government denied that the top-secret aircraft - nicknamed Nighthawk - existed.

Then, in 1991, 40 Stealth fighters were suddenly deployed for action in the Gulf War.

Ranging the night skies over Baghdad on 1,270 missions the Nighthawks struck the most heavily defended Iraqi targets to stunning effect.

Now from the cloak of X-Files denial comes a Stealth successor: more powerful, blacker, faster and even more secret.

Under the codename Project Aurora - which may be a wrap for several secret aircraft - the planes are classified within the US defence department's black programme - one whose existence is not admitted by the authorities.

Experts claim experimental and prototype Aurora aircraft are using Scotland, the skies above the North Sea and the wilderness areas of far-Northern Europe as their testing ground.

Bill Sweetman, former technical editor for Jane's Information Group and an author of three books on Stealth technology claims the areas are ideal proving ranges.

"It certainly keeps them out of the eyes and ears of the US observers," he said.

He claims that after 17 years the US defence department is reaching the latter stages of trialing space-age military aircraft capable of astonishing speeds.

"There continues to be a huge black hole in what we know the Pentagon has spent money on," he told the Press and Journal.

"In 1999 black projects accounted for £12.1billion of USAF research expenditure - that is almost 40% of the £32billion research and development budget."

Advanced secret aircraft developed at highly classified Government facilities in the Nevada Desert almost certainly include both manned and unmanned hypersonic jets designed to perform strategic reconnaissance and other less conventional missions for the US Air Force and its NATO allies.

A number of these aircraft have been seen and heard by ground-based and airborne observers in the western USA and in northern Europe during the past 10 years.

Based on more than 60 eye-witness reports there appears to be at least three distinct types of vehicle:

One is a "triangular-shaped quiet aircraft" observed with a fleet of Stealth fighters several times between 1989 and 1995. This may be a demonstrator or prototype of the much vaunted McDonnell Douglas A-12.

Another is a high speed aeroplane characterised by a very loud, deep rumbling roar, reminiscent of heavy-lift space rockets. In flight it makes a pulsing sound and leaves a segmented vapour trail.

The final contender is a high altitude jet that crosses the night sky at extremely high speed and at altitudes in excess of 50,000 feet. It is usually observed as single bright light but no engine noise or sonic boom is heard.

Observations are augmented by many reports of low-pitched, rumbling sonic booms.

In one seven month period a small team of observers in California logged at least 30 sonic booms believed to be produced by the same unknown aircraft.

Claims have surfaced that booms from Aurora test flights are responsible for sudden avalanches in Norway and an earthquake in the Netherlands as well as unexplained radar blips, eerie noises and isolated UFO sightings in Scotland.

Reporters from Norwegian newspaper Aftenposten say they have received numerous complaints of sudden bass-like booms from isolated fishing communities and farmsteads between Trondheim and Narvik, followed by sudden avalanches of snow.

They say that recently released information suggests that a 1986 avalanche in the Troms area of northern Norway, which killed 16 NATO soldiers, may have been triggered by early tests of a secret supersonic jet.

The P and J understands that Norwegian Government officials are now concerned about Aurora flights, damage caused by sonic booms and the lack of consultation from their US NATO allies.

An Oslo-based Government spokesman said they were carrying out long-standing research into the causes of avalanches and they were aware of the concern over test flights by military aircraft from the UK.

"We always receive a number of complaints from people over low-flying aircraft and sonic booms - and it is probably true that some sonic booms cause avalanches," he added.

However, he refused to comment on the existence or activities of the Aurora.

Dutch scientists have meanwhile blamed the secret jet for causing a sudden earth tremor which jolted the north coast of the Netherlands.

A North-east RAF base recently traced a very fast radar blip across the North Sea. But when the incident was reported to RAF Buchan, superior officers denied all knowledge of it.

Oceanic Air Traffic Control at Prestwick also tracked fast-moving radar blips. It was claimed by staff that a "hypersonic jet was the only rational conclusion" for the readings.

Experts claim the Aurora has probably flown out of RAF Machrihanish airfield in Argyll while hi-tech tracking equipment at Benbecula, RAE West Freugh in Galloway and Fylingdales monitor its progress.

There have been reports of unidentified night-time aircraft noises from Machrihanish for a number of years.

But with the Kintyre base now downgraded to a care and maintenance position, experts are puzzled about the location of the Aurora's new European test base.

Maryland journalist Lee Hickling has studied Aurora sightings in great detail.

"The information currently available shows Scotland and the North Sea are used extensively for the testing of these aircraft," he said.

Mr Hickling, who for nine years covered science and manned space for the Gannet Newspapers Washington bureau, added: "I believe it is extremely likely that the aircraft - test beds for hypersonic engine and control technology - would be unmanned, because human bodies could not stand the G forces generated by manoeuvres at hypersonic speed."

But last night Bill Sweetman said high speed - such as at Mach 7 or 8 - would not exclude manned aircraft. "It is only when you manoeuvre an aircraft at that speed that G forces come into play," he said.

Mr Sweetman said the development of the Aurora within the US defence department's "black projects" was a natural progression from the Stealth fighter, which first flew in 1982.

"They would not have sat still for 17 years," he said.

"The evidence is strong that high speed propulsion and aerodynamics are at the cutting edge of this new development and the long runways at Groom Lake (USA) and Machrihanish would be ideal to fly the plane from."

He said the skeleton staff at the "care and maintenance" RAF Machrihanish would be a perfect cover for further trialing of Aurora aircraft.

"One of the missions of high altitude supersonic aircraft was to operate over the North Atlantic as a reconnaissance strike system against the Soviet Northern Fleet and it would be natural to continue that test range despite the end of the cold war."

The ultimate in aerodynamics the aircraft could reach anywhere in the world in three hours, he claimed.

However an MoD spokesman said last night: "There are no United States Air Force prototype aircraft based at British airbases and no authorisation has been given by Her Majesty's Government to the USAF - or any other US body - to operate such aircraft within or from the United Kingdom.

A spokesman for the US defence department denied any knowledge of Aurora or "Deep Black" aircraft. Mr Sweetman said it was natural for British and US military spokesman to deny the existence of the plane.

"Put it this way," he said, "In 1988 the US Air Force had 50 F-117 Stealth aircraft operating in Nevada and still denied they existed."