The journey two naval officers made some time ago to the very deepest point on the earth makes us realize how much of the world still remains to be explored.The two men went down 7 miles to the bottoms of the Pacific Ocean inside a small steel ball called a "bathyscaphe" to find out if there are any ocean currents or signs of life.Above the ball there was a special tank full of petrol which was slowly emptied into the water to make the bathyscaphe heavy enough to complete its journey. It was necessary to set out early, so that the bathyscaphe would come to the surface in daylight, and so be easily found by the mother ship which would be waiting for it.The divers began preparations at dawn and soon afterwards, when all was ready, the steel ball disappeared under the surface of the water. The divers felt as if they were going down steps as they passed through warm and cold layers of water.In time, the temperature dropped to freezing point and the men shivered inside the ball.They kept in touch with the mother ship by telephone describing how they felt.Then , at a depth of 3,000 feet, the telephone stopped working and they were quite cut off from the outside world.All went well until some four hours later at 30,000 feet, the men were startled by a loud, cracking noise; even the smallest hole in the ball would have meant instant death.Luckily, though, it was only one of the outer windows that badly broken.Soon afterwards, the bathyscaphe touched the soft ocean floor raising a big cloud of "dust" made up of small, dead sea-creatures.Here, powerful lights lit up the dark water and the men were surprised to see fish swimming just above them quite untroubled by the enormous pressure.But they didn't dare to leave the lights on for long, as the heat from the lights made the water boil.Quite unexpectedly, the telephone began working again and the faint but clear voices of the officers were heard on the mother ship 7 miles away.After a stay of thirty minutes the men began their journey up, arriving three hours later, cold and wet through, but none the worse for their experience. According to the passage the two naval officers stayed in the water about   _  hours during the whole journey.
3 4 7 8