The monsters that emerged from beneath the deckchairs

For years, tourists have flocked to the holiday resorts of the Caribbean and the Mediterranean, relaxing on the sandy beaches in blissful ignorance that fantastic sea creatures, never before seen by man, are lurking silently beneath their deckchairs in underground caves.

But the tourism trade in resorts such as Majorca, Ibiza and the Bahamas threatens these unique animals, whose habitat is below the volcanic islands, researchers claim.

Geoff Boxshall, of the Natural History Museum, has discovered huge varieties of sea life, including crabs, shrimps and sea-lice, which have followed their own evolutionary paths after being trapped thousands of years ago when the caves formed on the ocean floor.

Reaching the partly flooded caves, which are cut off from the open sea, can take up to eight hours' swimming. But once inside, "seeing is believing", says Professor Boxshall. The caves range from a few metres high up to 20 metres, interlaced with huge stalactites and stalagmites, and the water contains an amazing diversity of life.

However, the increasing demand on the islands for fresh water, sewage outlets and amenities such as golf courses could destroy the creatures, which offer important clues to continental drift and how evolution occurs in isolated groups of creatures.

"There are fantastic animals there which occur nowhere else on Earth," Professor Boxshall told the Festival of Science yesterday. "They only separated from the rest about 12,000 years ago. That's a short time in the scale of evolution. They're unique, and need to be protected."

The animals represent more than one entirely new genus - a wide-ranging biological classification. (Domestic cats and lions, for example, belong to the same genus, Felis.) By finding the nearest relatives of the animals, scientists can track how the islands moved with continental drift - in some cases, to opposite sides of the Atlantic.

But increasingly, the caves are under threat. One was discovered by hotel workers who were digging a sewage outlet. Others are being affected by the extraction of water for desalination to supply the burgeoning tourist trade on the islands. "The scale of removal of groundwater has resulted in the penetration of sea water far into the centre of the Mallorca," Professor Boxshall said of a recent discovery on the island. That contamination could kill the unique crabs, which have grown used to fresh water. Elsewhere, the building of a golf course threatened to fill in an underground cave with rubble.

The creatures were trapped in the caves as the islands were formed in bursts of volcanic activity on the seabed. They then evolved to survive in darkness and in water with low levels of salt and dissolved oxygen.

Professor Boxshall hopes that the tourism industry might be persuaded to exploit its biological treasure rather than plundering it. Some of the caves have been marked for preservation, while other better-known ones have been opened to the public.