stumbled upon a hissing and oozing metal object embedded in a rocky cliff at Castle Island.

Fearing it was an olden-day cannon ball about to explode, Mr. Steven DeSilva wasted no time hoisting himself up and fleeing to the other end of the island.

He radioed his boss, Government Conservation Officer Dr. David Wingate, who summoned the Police and advised him to get off the island immediately.

Dr. Edward Harris of the Maritime Museum was expected to motor out to the island today to examine the mysterious object.

Mr. DeSilva had been abseiling off the north face of the island at about 11 a.m., checking Longtail nests and getting rid of casuarina seedlings to prevent them uprooting the rocks.

Dangling some 35 feet down the cliff, he came upon what he thought was a rusted rock.

"It looked a little strange so I chopped at it with my hatchet,'' he said.

"The core of it fell away and exposed what seemed to be a cricket-ball sized cannon ball. It had a crack in it which began to hiss and ooze liquid. I very quickly put it back and fled.'' Mr. DeSilva said he thought it was an old piece of ordnance that had not yet exploded.

"No one needed to tell me to get out of there,'' he said.

The Police officers who arrived on the scene could not get to the object as they had no equipment to abseil.

However, a sergeant well-informed in explosives guessed Mr. DeSilva had probably found an old cannon and that the hissing and liquids were created when the vacuum it was sealed in broke.

Police spokesman Sgt. John Dale said last night it was "absolutely no danger'' to anyone and there was no chance of it exploding.

But Agriculture, Fisheries and Parks director Mr. John Barnes was taking no chances.

"I certainly wouldn't advise anyone going onto the island until we know what it is,'' he said.

Mr. Barnes said it could contain poisonous gases or be a First World War bomb with a time fuse.

However, he said, it was more likely a cannon ball from this century, probably fired from an old shooting range at Cooper's Island.

"The fact it seemed to be hollow suggested it was a modern-day cannon,'' he said. "The old ones were made of cast iron.''