Have you ever noticed that, in the summer, when your sweat dries it will leave marks on your T-shirt? The marks are tiny bits of salt.

If you go to the seaside, you will know that seawater does the same to your clothes, because it also has a lot of salt in it.

Water evaporates from the surface of the sea but the salt remains¹. Have you ever thought about whether the seas will keep getting saltier? The answer is no. The oceans have stayed at about 3.5 per cent salt content for hundreds of millions years. It maintains a constant level of salt in different ways.

Pick up a clamshell and weigh it in your hand. It is heavy. All creatures need sodium³ to live and most need calcium⁴ to build bones and shells. The clam², like all sea creatures, gets its sodium and calcium from seawater. When creatures die, their salt is locked up in bones and shells, which drop to the bottom of the sea.

Reactions between seawater and rocks are not just one way. Sea salt not only dissolves from rocks, it also reacts with the rocks of the ocean crust and volcanic⁵ lava⁶. The reactions take some of the salts in seawater away from the sea.

The outer hard crust of Earth is made up of a dozen or so hard plates that drift on extremely hot, soft rock like floating islands on a sea. The heat within Earth is not the same everywhere and the plates move due to the heat.

When an ocean plate bumps into a land plate, the less dense⁷ land plate floats over the ocean one. The ocean floor gets pushed under, and its salty rocks, along with shells and bones, are lost deep within Earth.

So, that's why the seas are salty but don't get any saltier.

你有没有注意到，夏天的时候，汗水干了之后会在T恤衫上留下一些痕迹？其实，这些痕迹是由极少量的盐留下来的。

海水含有大量的盐分，因此也会在人的身上留下同样的痕迹。

海水蒸发掉了，而盐留在了海里。你有没有想过海水会变得越来越咸？答案是否定的。亿万年以来，海洋的盐度始终保持在3.5%左右。海洋恒定的盐度是由诸多原因造成的。

捡起一个蛤壳，在手里掂量一下它有多重。感觉很重。一切生物的生存都需要钠，而且大部分生物都需要钙质来生成骨骼和贝壳。如同所有的海洋水族一样，蛤从海水里摄取钙质。而水族们死后沉入海底，它们体内的盐分就被固定在骨骼和贝壳里。

海水和岩石之间的化学反应并非是单向的。岩石溶解产生海盐，而同时，海盐又与大洋地壳中的岩石和火山熔岩进行化学反应，从而把一部分盐分从海水中带走。

地球外部的坚硬外壳由大约12个板块构成。就像在海上漂流的岛屿一样，这些板块在极端高温的地下岩浆上漂移。地球内部的热量在各个地方不均匀，由此导致了板块运动。

当大洋板块与大陆板块碰撞时，稍薄的大陆板块就会漂在大洋板块之上。这样，大洋底受到了挤压，含有盐分的岩石，连同生物的贝壳和骨骼，被埋在了地球深处。

所以，这就是咸的海洋不会变得更咸的原因。