Sunless Sea (opens in new tab) is getting its first piece of downloadable content, which won't be set on the vast subterranean ocean that is the Unterzee, but under it—in the under-Unterzee, if you will. Zubmariner provides you with a, er, 'zubmarine', and gives you a bunch of new "story-laden ports to visit and zee-beasts to encounter".
Zee-beasts like this scary fella, who "can move rapidly, ram, and expectorate gobbets of vile fluid", like your mum can.
Other Zubmariner delights include the underwater city of Wrack, made out of shipwrecks, along with various bio-luminescent creatures and pretty coral. Your vessel's prow light isn't powerful enough to illuminate everything on the sea bed, so expect certain things to have a pop at you from the darkness, which is a bit of a scary thought.
There's no date yet for the Zubmariner DLC, but you can get an early look at it on Failbetter Games' site (opens in new tab).
Disclaimer: PC Gamer contributor Richard Cobbett wrote parts of Sunless Sea, and presumably has something to do with Zubmariner as well.