With the dress, a diver can work comfortably at one hundred to a hundred and twenty-five feet, but men who know the fisheries doubt if that can be exceeded. Nor does it seem needful to go deeper, for in seas which have been explored at greater depths it is usually found that the bottom consists of ooze unsuitable for the life and growth of the oyster. Beyond those inherent to the art of diving, either method has its peculiar difficulties after bottom is reached. In naked diving, especially at the shoals of Ceylon and Venezuela, where the shells are small and abundant, it is simply a question of gathering as many as possible while the breath lasts and looking out for the dangerous fishes indigenous to tropical waters. Sharks are common in many of the pearl-oyster seas, but experienced divers do not fear them greatly, as the fish, formidable as it may appear, and dangerous as it is when it can come upon one unawares, is easily frightened. Many expert swimmers of the Indian and Pacific oceans do not hesitate to attack them in their own element. Usually vigorous splashing will frighten them away. The dress-divers of Australia scare them off by allowing a jet of air to escape. As the bubbles start for him, the man-eating monster shoots away from them as if terror-stricken. The diamond-flounder of the Pacific and Indian oceans, a huge flat fish with a habit of seizing its prey between the side fins and crushing it, is more dangerous. If a dress-diver of experience sees one of these approaching, he is apt to shut off the air-escape of his helmet and signal to his tender that he is coming to the surface as fast as he can get there. The rock-cod also is sometimes troublesome on the Australian coast. Occasionally he attains an enormous size. This fish lies hidden in submarine caves, his head protruding and his monstrous jaws yawning vertically wide like an entrance to the cave itself. But accidents from the denizens of the sea are comparatively few; the physical results of deep-sea diving are more to be dreaded, for paralysis hovers close to the thirty-fathom line. Although dress-diving has the advantage over naked diving that it gives a supply of air to breathe while at work, it also entails dangers and difficulties from which the old method is free. An imperfect supply of air may cause the bursting of a blood-vessel. Fouling of the lines might not only cut off the air supply entirely, but prevent the man, anchored by his heavy dress under twenty fathoms of water more or less, from signalling the man at the life-line. As on dry land, there are holes and precipices at the bottom of the sea to be avoided.