When a Swedish ship that sank in 1628 was recovered from the port of Stockholm, historians and scientists were overjoyed with the chance to examine the remains of the past. The ship construction showed how ships were built and operated during the seventeenth century. In this way, artifacts, objects made by human beings, provided a picture of daily life almost 400 years ago. Underwater archaeology-----the study of ships, aircrafts  and human settlements that have sunk under large bodies of water-----is really a product of the last 50 years. The rapid growth of this new area of study has occurred because of the invention of better diving equipment. Besides the Swedish ship wreck ,underwater archaeologists have made more exciting discoveries such as the 5000-year-old boats in the Mediterranean Sea. Underwater archaeology can provide facts about the past. In ancient ports all over the world are ships sunken in the past 6,000 years. There are also sunken settlements in seas and lakes telling of people's way of life and their systems of trade in ancient times. Underwater archaeologists want to study these objects to add to the world's knowledge of history, but they have to fight two enemies. One enemy is treasure hunters who dive for ancient artifacts that they can sell to collectors. Once sold, these objects are lost to experts. The second enemy is dredging machines often used to repair ports. These machines destroy wrecks and artifacts or bury them deeper under sand and mud. By teaching the public about the importance of underwater "museums" of the past, archaeologists are hoping to get support for laws to protect underwater treasures. What is the main purpose of the passage?
To introduce a young branch of learning. To discuss the scientists' problems. To explain people's way of life in the past. To describe the sunken ships.