The Great Barrier Reef is made of hard corals.

大堡礁由硬珊瑚组成。

In the 19th century, settlers mined the island for bird guano, leaving the place mostly barren, hard coral.

19世纪，移民们在岛上开采鸟粪，留下了很多贫瘠坚硬的珊瑚。

Hard corals create the reef by secreting calcium carbonate—or limestone, a type of rock, which gradually builds up into a solid structure.

硬珊瑚通过分泌碳酸钙来造礁，或称之为石灰岩，一种岩石类型，逐渐形成固体的结构。