Salt in the sea comes mostly from minerals washed off rocks by rain and carried down rivers.

The longer version

Rainwater is mildly acidic because it picks up carbon dioxide while falling, and that weak solution attacks rock. Sodium, calcium and magnesium ions come loose during weathering, then rivers carry them seaward. Once there they have nowhere to go: the ocean is the end of the line, so dissolved material accumulates over immense spans of time.

Removal happens slowly, through burial in sediment, uptake by living things and trapping in pore water. Undersea volcanoes and hot springs add a further trickle from within the crust. Small ongoing inputs balanced against enormous storage keeps seawater steadily mineralised rather than continuing to concentrate.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.