One single reef, for instance, which is entirely made by them, stretches along the north-east coast of Australia for nearly a thousand miles.

Of this you must read some day in Mr.Jukes's _Voyage of H.M.S.

"Fly_." Every island throughout a great part of the Pacific is fringed round each with its coral-reef, and there are hundreds of islands of strange shapes, and of Atolls, as they are called, or ring-islands, which are composed entirely of coral, and of nothing else.
A ring-island?
How can an island be made in the shape of a ring?
Ah! it was a long time before men found out that riddle.

Mr.Darwin was the first to guess the answer, as he has guessed many an answer beside.
These islands are each a ring, or nearly a ring of coral, with smooth shallow water inside: but their outsides run down, like a mountain wall, sheer into seas hundreds of fathoms deep.

People used to believe, and reasonably enough, that the coral polypes began to build up the islands from the very bottom of the deep sea.
But that would not account for the top of them being of the shape of a ring; and in time it was found out that the corals would not build except in shallow water, twenty or thirty fathoms deep at most, and men were at their wits' ends to find out the riddle.