In daily life， we are accustomed to① measure the height of objects on land above sea level on the basis of sea level. In fact， sea level is not flat.
Why？ There are two main factors ： the first is raising tide②， ebb③ tide， storm and air pressure level and so on， the other is the different topography④ of seabed， both which make the sea level unflat.
As we know， the seabed topography is very complex， in which not only seamounts or flat plains， but also many steep submarine trenches⑤ are distributed. Therefore， a sea area may be a few meters or even 10 meters less than or greater than another area. According to altimeter measurements used with radar (radio)， scientists found that there is a different height in different sea areas of the Atlantic Ocean， even in the relatively small sea area between the islands of South Carolina and Puerto Rico. Generally speaking， the sea waters of seabed formed with a series of mountains is higher than other areas； oppositely， basin seabed is less. For example， both being within the Atlantic waters， there is a under sea waters of Puerto Rico sea where there is a concave⑥ is obviously lower than surrounding areas； otherwise， due to a 3500-metre-high Ridge under sea， the sea waters of Eastern Brazil is higher than other regions.
Moreover， sometimes the sea level is related with the giant mountains near or accumulation⑦ of material formed by mountains. The accumulation of this material makes the surface gravity bent， thereby creating a power， which drive water flows from one region to another.
Therefore， we have enough reasons to say that the sea level is not flat.
① be accustomed to习惯于，适应于
② tiden. 潮，潮汐，潮水［C］［U］
③ ebbn. 落潮，退潮
④ topographyn. 地志，地形图，地形学，地形测量学
⑤ trenchn. 沟，沟渠［C］
⑥ concaveadj. 凹的，凹面的
⑦ accumulationn. 积累，积聚；累积物，积聚的一堆
海平面是不是平的
我们常习惯以海平面为准来测量海平面以上陆上物体的高度。但事实上，海平面并不平的。
为什么呢？有两个主要因素：一是涨潮、落潮、风暴和气压高低等因素，二是海底地形的不同，这这两者都决定了海面的不平。
我们知道，在海底不仅分布有巍峨的海底山脉、平缓的海底平原，而且还有许多陡峭的海底深沟，地形十分复杂。因此，一个海区的海面有时会低于或高于另一个海区几米、甚至十几米。据科学家们使用雷达（无线电）高度计测量，发现在大西洋海面不同海域存在着高度差，甚至在美国南卡罗里州和波多黎各岛之间比较小的海域内也是如此。一般情况下，海底是一座山脉的地区，海面就比其他海域高一些；而海底是一个盆地的地区，海面就比其他海域要低一些。比如，同是大西洋海域，波多黎各海下是一片凹地，因而这一地区的海面就比周围地区明显的低；而巴西东部由于海下有一座3，500米的海岭，所以这里的海面就比其他地区要高。
此外，有时海面的高低还与附近的巨大的山脉或山脉所组成的物质的积聚有关。这种物质的积聚，可以使其表面引力弯曲，从而形成一种动力，驱使水离开一个地区而流向另一个地区。
因此，我们有充足的理由说，海平面往往是不平的。