Sunken Rocks, Shore Reef, Rock Reef
Noun
がんしょう
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.
A reef made of boulders is still a reef, but it's made of sunken rocks.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.