I’ve seen Mare Internum floating around recommendation lists for a while, and I finally delved in past the first few pages and holy shit it’s good.
A couple decades in the future, researchers on Mars are probing its caves for possible colony foundations. Michael Fisher, with plenty of his own problems, gets vastly in over his head and has to figure out just what the hell is going on here.
The author’s formal biology education really shines through: the Martian flora and fauna are brilliantly weird and well-realized, and this hits all my favorite benevolent-parasite body-mod notes.
I don’t want to say any more and ruin the joy of exploration, but if you love clever sci-fi and very troubled people dealing with bizarre worlds, dive in.