If you look at this diagram of a coral reef, we know a lot about that part up near the top, and the reason we know so much about it is scuba divers can very easily go down there and access it.
如果你看一下这张珊瑚礁的图表 我们对最上方的那部分了解很多 之所以如此，是因为 水肺潜水员能轻松地下潜到那里