"Caverns measureless to man..." Sunless Sea art direction
Recognisably connected to Fallen London. Which means cartoony but spooky, a style that FL players will already be familiar with. It means glowy things, silhouettes where appropriate, strong colours against dark backgrounds and textured, hand-painted feel.
Light and darkness. The Unterzee is dark by default, with minimal ambient light from the cavern roof. What light there is, you bring yourself. Ships have powerful searchlights (energised by heated glim) and strings of fairy lights to illuminate decks; creatures have adapted to darkness in manner of deep ocean dwellers; buoys light trade lanes; and so on.
A strong, simple colour scheme. Terrain and beasties need to be colourful to offset the slightly deadening effect of all that darkness, but we don't want a fruit salad effect, so we're sticking with a few simple hues - the tealy green of the ocean, the icy blue of glim-lights, the brass and iron tones of the interface.
And finally...
Subterranean: because of the vast scales involved, it's easy to forget that the Unterzee is actually an underground lake. The concept above, one of the earliest ones I did, is an attempt to remind ourselves of that, and in the final game you can expect to see stalagmites poking up from the ocean floor, giant geodes and - occasionally - the gigantic natural pillars that hold the cavern roof up. On a related note, much of the wildlife of the Unterzee is inspired by those wonderfully icky creatures that live at the very pit of the ocean, the isopods and angler fish and (of course) giant squid. Many of these creatures have been around since long before the Fall, and they've adapted to the lightless conditions, either hunting though other senses than sight, or providing illumination of their own. We'll have more on them in a later post.