Reef
Sunken Reef
noun
A dark reef is still a reef. It's just really deep underwater so it's kinda dark under there. Because of that it can also be called a sunken reef.
あんしょう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
船長は、暗礁に気をつけてと言って、僕に操縦を任せた。
The captain told me to be careful of reefs and then left the steering to me.
捜査は暗礁に乗り上げてしまったな。
The investigation went cold unexpectedly.