Board Thread:Questions and Answers/@comment-24731329-20150303101022/@comment-25806814-20150303142548

It's the boss HP bar. Unlike previous map, extra operation require you too kill the boss 4 time to clear it.

Every time you sink the boss (first ship at the boss node), the bar will decrease. And after the 4th kill it will explode, then you will get a medal as reward.