the British Association festival in Newcastle

A ROYAL Marines officer saved 20,000 civilians from drowning at the

hands of Serbian dam-busters, it was revealed yesterday.

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The unsung hero, Captain Mark Gray, 29, from Devon defied land mines

and booby traps to open a sluice gate on top of the Peruca dam in

Croatia shortly before the occupying Serbs detonated explosives deep

inside it.

His audacious feat two years ago has been unknown to the public until

described to the Science Festival yesterday by engineering Professor

Paul Pack from Oxford University.

He described how Serbian militia had expelled UN observers from the

65-metre-high dam in January, 1993, and set off huge explosives in a

maintenance gallery that ran the dam's length at foundation level.

''This was an attempt to use the 540 million cubic metres of stored

water as a weapon of mass destruction to the downstream land and

population,'' said Professor Pack.

''Some 20,000 people would have been drowned or rendered homeless had

the dam failed as intended.''

Severe damage was caused to three points in the dam corresponding to

where the saboteurs had placed their explosives. In the central section

alone it was estimated that 15 tons of explosive had been used.

At each of these three points the top of the dam -- made of rock fill

with a clay core -- sagged by two metres, said Professor Pack, who was a

member of a British team despatched by the Overseas Development

Administration to inspect it and advise on repairs after the Croatians

reoccupied it.

''During the tenure of the UN observers, but while the dam was in Serb

hands, the British major, who had trained as an engineer, had visited

the site and observed that the Serbs were holding the water level well

above the correct full supply level,'' he said.

''On his own initiative, and exceeding his authority, he opened the

surface spillway gate sufficiently to slowly reduce the water level. He

managed to lower the water level by some metres by the time the attempt

to destroy the dam took place.

''Had he not been able to reduce the level, there is no doubt that the

dam would have failed as water would have poured over the slumped crest

after the explosions.''

As it was, Professor Pack said it was only a miracle that the dam had

not failed. With gunfire echoing in the hills engineers had to race

against time before the ongoing erosion of the dam's clay core caused a

blow-through and total collapse.

Professor Pack said he learned later that the officer could have been

disciplined for exceeding his authority.

''I wrote to the Ministry of Defence and told him he should be given a

medal instead.''

Earlier, it was learned that drug experts have closed a loophole that

was enabling athletes to cheat with impunity.

Extensive research on natural levels of the hormone

dihydrotestosterone will allow sports authorities to make charges stick

against athletes who boost their levels artificially.

Dihydrotestosterone is a metabolite of the male hormone testosterone;

it occurs naturally in males and, to a lesser extent, in females, but in

both cases the levels vary from one individual to another.

Dr David Cowan, from the Drug Control Centre at King's College,

London, said: ''Thanks to extensive population testing we now have the

data setting out the parameters for its natural occurrence. This has

enabled us to set threshold levels beyond which we can say with

authority that the levels have been boosted artificially, and the

disciplinary bodies will have the data to back them up.''

The use of dihydrotestosterone came to prominence last year when

Chinese swimmers accused of using it were disqualified from the Asian

Games.

It binds to receptors in the muscle, giving added size and strength,

in the bones, boosting the manufacture of red blood cells, and in the

brain, where it arouses a more aggressive and competitive edge.

It can be obtained in the form of a fine powder, mixed with whisky,

and rubbed on to the chest, where it will penetrate the skin, said Dr

Cowan.

Although it does not produce acute toxic effects, long-term use can

lead to kidney and liver damage.

But higher levels, within the normal parameters, may be good news.

Tests on Greek army recruits, published recently in the Lancet, showed

that the higher the levels they had the more orgasms they achieved in

the course of a week.

Meanwhile, Dr Cowan and his colleagues have turned their attention to

other cheating agents which have proved difficult to detect, and hope to

crack them too.

In another medical breakthrough, saliva tests may soon be used to

determine when workers are over-stressed and need a break.

The tests will go under trial next year on business executives and

footballers.

Excessive stress can lower resistance to infection, said former

athlete Dr Lynn Fitzgerald, Reader in Sports Science at Brunel

University.

''Accountants are prone to herpes towards the end of the financial

year when they are struggling to get their books balanced,'' she said.

''Athletes are pushing themselves harder than ever before, and more

are suffering from illness -- colds, sore throats and flu, post-vital

fatigue syndrome, and chronic fatigue.

''Infections result in poor performance, with weeks or even months of

training lost.''

Although some stress was no bad thing, there was a well-established

link between psychological stress and immune competence, she added.

Cortisol, adrenalin, and neuropeptides are released during

psychological stress as well as during physical exercise.

''The effects on the immune system of the physical stresses of elite

athletes appear to be cumulative,'' said Dr Fitzgerald. ''Each

exhaustive training session or competition compromises the immune system

for a few hours, which provides an opportunity for infections to take

hold.''