World 2, Map 3 (2-3, Orel Sea). Useful for gathering resources if you sortie with an all-submarine fleet.
And because it's so useful for resources, it's notoriously run repeatedly with submarines to the point that it's a meme. See the wiki on orel_cruise for more info (or for that matter, the other images under the tag).
Akatsuki and Hibiki felt an unimaginable sense of fear.It's all thanksHey, Ro-chan!I did my best, yes!Yes?Oh?Ro-chan, wasn't your skin all white before this?I've been wondering how did you manage to get such a beautiful tan.to the Orel Cruise!I'll ask Ro-chan how to get a tan!At first it's only for the daily quest "Seize Control of the Southwestern Archipelago Sea!", but there are times when I have to Orel cruise alone as well, yeah. When our resources were depleted during events, for some reason Ro-chan was asked really hard to do it again and again and again and again throughout the day.