KanColle Kai talk:FAQ/@comment-49.150.67.66-20160316161454

Q: How can we stop the Abyssals from recapturing a red hex?

A: If it seems you can't reduce the threat level of a hex and it is at its red level you must have at least one combat fleet at that hex before you end your turn. This means a fully restocked fleet in the hex. At the start of your next turn your assigned fleet would enter that map's counter attack map and you must fight your way towards the Abyssal fleet's command node, appearances may vary but it looks like an anchor in a middle of a sea turning red. Like any other boss nodes it may drop unbuildable Kanmusu and the like.

Defeating the Boss Node clears the hex though it doesn't mean the hex is cleared of any maps on it you haven't finished up to that point.