Submarine 707 v3
OctonionicThe first story of Submarine 707 closes with lots of explosions. We now move into the second story, which is the one that was adapted in the 1997 OVA. The adaptation is very loose, so don't expect more than a handful of plot points to line up if you happen to have seen it.
The continent of Mu is treated a bit (just a bit) more seriously by the Japanese than it (or Atlantis) is by English-speaking world. To the West, Mu is mostly considered the invention of James Churchward in the 1930s, and has been relegated to fiction of the more fantastical variety. In Japan, however, the idea gained a bit more traction, possibly because Churchward described the Japanese as descended from Mu's rulers, possibly because several Japanese documents older than the 1930s describe some sort of lost civilization in the Pacific, and possibly because of things like Yonaguni. The idea has survived, in heavily modified form (actual lost continents are pretty much ruled out); for example, Japanese Wikipedia at present mentions a theory linking Mu with the Tongan Empire.
The informational sidebar sections in this volume start moving beyond the current state of submarine technology into what Ozawa predicts the future might be like. I may speak only for myself, but it's a lot of fun to see the “manga authorship as chance to talk about one's hobbies” phenomenon in earlier days.
As a side note: it may seem as if every officer in this manga has the title “captain”, but it's a bit more varied in the original. There's 艦長 (kanchou), “one in command of a military ship, regardless of rank”, then 船長 (senchou) and 機長 (kichou), for non-military ships and aircraft respectively. Hayami's actual rank is 一佐 (issa), meaning “Captain”, and Red's rank is 大佐 (taisa), the equivalent rank from earlier decades. To round it all off, Blood occasionally uses キャプテン (kyaputen), the transliterated word “captain” from English.
Volume 3