Reef
Sunken Rocks, Shore Reef, Rock Reef
noun
A reef made of boulders is still a reef, but it's made of sunken rocks.
がんしょう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
岩礁を埋め立て、人工島に造り替えたんです。
The reef was filled up and replaced with an artificial island.