Land, sometimes referred to as dry land, is the solid surface of the Earth, that is not covered by water. The division between land and ocean, sea, or other bodies of water, is one of the most fundamental separations on the planet. The vast majority of human activity has historically occurred, and continues to occur, on land. Furthermore, the life forms that exist primarily on land, including terrestrial plants and terrestrial animals, differ from those that exist primarily in large bodies of water. The demarcation between land and water varies. In some places, it is sharply defined by solid rock landformscoming directly to the edge of the water. In other places, land may slowly give way to marshes or swamps, with no clear point at which the land ends and a body of water has begun. In such places, the demarcation can further vary due to tides and weather, with the area where land and sea interact being called the "coastal zone".