Atoll
Circular Coral Reef
noun
A reef in the shape of a loop is an atoll. If you don't know what that is, it's a circular coral reef.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
その時、日本のマグロ漁船「第五福竜丸」がたまたまビキニ環礁を航行しており、乗組員が被ばくしてしまった。
The Japanese tuna boat "S.S. Lucky Dragon 5" accidentally sailed around Bikini Atoll and the crew was exposed to radiation.