For centuries Dutch engineers have been fighting a war against water. Their main enemy is the sea. A large part of the country is below sea level. In fact, Holland is also called the "Netherlands" which means "low land. " Very tall and strong walls, called dykes, have built to keep out the sea. But in very rough weather the sea may suddenly burst through a dyke. Great damage can be caused by floods when this happens. Three large rivers are also part of the problem. They flow through Holland into the North Sea. They have cut many passages across the country. At low tides, the rivers flow into the sea as usual. But at high tide, the sea can flow into the rivers! This is because the land is so low. The battles against the water never end, but they have made Holland a bigger and bigger country. In order to prevent floods, the engineers take, or reclaim, land that was under water. The Dutch have been reclaiming land for seven hundred years. Land is usually reclaimed from a passage between islands. Two dykes are built across the passage, so they block the water between them. Then the engineers dig long canals and pump the water into them. At low tide, the canals empty the water into the sea. Because the land is so low, water from the sea and rivers can rise up through the ground. For this reason, the pumps continue working. The dykes contain gates, and through these the water is pumped out. In many parts of Holland, pumps must be working all the time. If they stopped, there will be very bad floods. A piece of reclaimed land is called polder , and Holland has thousands of polders. Some are very large, but others are quite small. There are farms on many of the polders. Corn and other crops grow well on the drained land, and it gives good grass for cattle. Many small towns and villages stand on polders. Even Amsterdam’s great airport was built on reclaimed land.If pumps stop working there will be very bad floods because______.