Saltwater lake on Mljet

Marine expert Dusan Zavodnik told BBC News Online that this coral reef was the largest not only in the Adriatic but in the whole of the Mediterranean - spreading across an area of 800 square metres.

Mr Zavodnik, who is leading the team of scientists, said it provided a unique marine habitat for many rare fish and sea species.

But he said the team had found a tangled, rusting network of undersea cabling and pipelines which had cut into the coral.

Croatian television showed pictures of the damage and said that the network of electricity and phone cables as well as water mains had been laid over the coral reef during the 1960s by the then communist authorities.

If we now try to remove the cables we may only inflict further damage

Vladimir Onofri

It said little thought was given to environmental protection in those days.

Many of the world's coral reefs are under threat from environmental damage, human activity and climate change.

The problem for the Croatian scientists is that any attempt to undo the damage on Mljet could make things worse.

Cables laid across the coral are rusting

Marine scientist, Vladimir Onofri, told Croatian TV that now the damage was done it might be better to leave the reef alone to avoid further damage.

The only good thing that could come from this now, Dr Onofri said, was to learn from past mistakes in order not to repeat them again in the future.

"If we want to continue to enjoy this natural treasure, then we must protect it," he added.