The city began as the capital of Louisiana (New France), part of the first French colonial empire at the mouth of the Mississippi River. It became a territory of the United States when President Thomas Jefferson bought the Louisiana Purchase in 1803. It became one of the world's great seaports. All the land is low, originally just a short vertical distance above sea level. In the last three hundred years, the city has sunk slowly into the marshy soil. Large portions of New Orleans are now below sea level. A system of many pumps, dikes, sea walls, and levees were built.