I don't really want to knock Deep Carbon Observatory, but since this was the review several people asked me to do...in the interest of reviewing things honestly: I just don't like this one.
You might like it; a lot of people seem to. I'm not into Deep Carbon Observatory for two reasons, mostly:
The first reason is that Deep Carbon Observatory is definitely what I term a miserycrawl. A dam has broken, leaving the world in the wake of a cataclysm, but there's treasure to be found if you venture into the depths of a murderhole. The people and creatures you meet on the way are invariably dying, starving, drowning, or being devoured. Alternately, or additionally, they are hostile, uncaring, duplicitous, or at best ungrateful for any help you give them.
This is where the devotion to a bleak mood in the adventure diverges from what the people I play games with enjoy over the long haul; in my experience, a scenario that relies on misery loses its punch pretty quickly once the players figure out that most everything they encounter is going to be crapsack-flavored. If it's orphans and cannibals all the way down, there's really no reason for them to care and every reason just to disengage from the game itself.
Other people can't get enough of this stuff, and that's fine, but it's not for me. I like dark elements to be prominent in campaign settings and adventures, but I prefer things to be a bit more subtle, played for fun, or at least tempered by other thematics.
I was discussing Deep Carbon Observatory with a friend who characterized its encounters as "vignettes," which stuck me as apt and makes me wonder if the adventure is intended more to be read than played; not much thought seems to have been put into how Deep Carbon Observatory will be used at the table. This lack of usability is the second reason I can't really recommend this book. The keyed descriptions aren't as terse or helpful as they could be, there is no overview for the adventure so the DM will likely need to take notes to prep it, the writing is a bit self-indulgent in places, the mechanical widgets are sometimes fiddly-yet-vague in that particularly "auteur OSR" way, and the maps look like this:
The art looks like this sometimes:
There is a random table that tells you to roll a d10. It has 12 entries, and the relatively short table hasn't been fitted to a single page:
Here's a room description I plucked out at random to illustrate what I'm talking about:
1. A knife room! Literally edgy! And it is designed so that some jerks can watch people suffer and die! 2. The room description isn't really helpful for conveying this information to the players. (How are they supposed to glean that it was a "cheap method of execution," or that it was meant to be a Chuck E. Cheese of murder-amusement?) 3. That stray apostrophe makes me sad.
Sometimes my two reasons for not feeling the Deep Carbon Observatory experience combine:
I can understand, and even empathize with, a certain single-mindedness when it comes to evoking a bleak mood or creating an atmosphere of dread, but reading Deep Carbon Observatory gave me the same feeling I get from a lot of the black metal theory stuff like Hideous Gnosis: clarity, purpose, and general usability shouldn't be sacrificed to ~poetics of ultima grimdark~. I can appreciate the effort that goes into making a thing, but ultimately this wasn't to my taste and isn't something I'd run.