A delta is a vast, fan-shaped creation of land, or low-lying plain, formed from successive layers of sediment washed from uplands to the mouth of some rivers, such as the Nile, the Mississippi and the Ganges. The nutrient-rich sediment is deposited by rivers at the point where, or before which, the river flows into the sea. Deltas are formed when rivers supply and deposit sediments more quickly that they can be removed by waves of ocean currents. The importance of deltas was first discovered by prehistoric man, who was attracted to them because of their abundant animal and plant life. Connecting waterways through the deltas later provided natural routes for navigation and trade, and opened up access to the interior. Deltas are highly fertile and often highly populated areas. They would be under serious threat of flooding from any sea-level rise.