This last part of his adventure made young Hassler very angry because it was so unscientific. He realized that the safety of all ships depended on accurate maps, called charts, of the coasts and harbors. Soon after he landed he began to make plans for a survey of the whole American coast. He talked to President Jefferson who agreed with him, and Congress finally gave him the job. At last his good charts began to help save lives. Today the United States Coast and Geodetic Survey carries on the work Hassler started. Using many ships and small boats with marvellous equipment, the scientific men of the sea go about their important and often dangerous work. It’s their job to map the earth that lies under the oceans, rivers and harbors. Here are some of the things they do along the Alaskan coast. A survey ship moves through the water sending sound waves to the bottom of the ocean. A delicate machine records the echoes made when the sound waves bounce back from the deep valleys and the high mountain tops that lie beneath the surface. Scientists know how fast sound travels in water, so they can tell exactly how deep it is. But some of the peaks are narrow and sharp. Even the wonderful machines miss them. And so these very modern vessels must do something old-fashioned and simple. Two of them travel side by side with a long wire cable hanging between them. If the cable catches on a rock, the men know they have snagged a sunken mountain top. Often, men on shore help the men on ships with their surveying. The instruments they use are so delicate that the warmth of direct sunlight would cause inaccuracies. One slight error might mean a shipwreck. So, even in Alaska, a surveyor works under an umbrella. [Illustration] You might think that all the charts and maps should have been finished in the long years since President Jefferson’s time. But the work of making charts can never be finished. The coastline is always changing. Currents and tides, storms and floods shift millions of tons of sand near the coast. Earthquakes and volcanoes raise land or lower it. A place that was safe for ships yesterday may be dangerous today. In Alaska, glaciers that run into the sea grow bigger or melt back. Sometimes these enormous rivers of ice push themselves out under the water. The little survey vessels mapping the ocean’s bottom have to sail over the sunken glaciers. There is always danger that, at any moment, a great mountain of fresh-water ice may break loose and rush toward the surface. When this happens, any vessel nearby is almost certain to be destroyed.