Deep Carbon Observatory is very rich on atmosphere. So much so that "the dungeon" of the adventure only sets up a slight portion of the adventure. Combat definitely takes a back seat in this adventure, but this isn't to say that it is non-existent or that there is very little threat to the PCs. In fact there is quite a lot of threat to the PCs.
The book is laid out rather organically. As GM you are introduced to things in much the same way and pace as the players are. As a result it is a rather "fun" read that had me asking questions and wanting to keep reading to find the answers until the very end where a handy timeline of events serves to tie things together as a sort of "big reveal" The downside of this is that it is not a quick and easy pick up and run adventure. Even getting to see "the big picture" as a GM, nothing is outright told to you, you have to figure out a lot of the connections of things on your own. This is definitely an adventure where as GM you are going to have to be taking notes before, during, and after the adventure. After? I hear you ask. Yes, the adventure has some definite world affecting consequences. On the bright side, most of these effects really only occur if the players choose to not be actively engaged in the adventure. There are several really clever ideas here, but the big take away is that the adventure does not exist in a vacuum frozen in time awaiting the adventurers to awaken it. The adventure is very organic and could very well resolve itself without any player involvement.
Art is some nice pencil work that conveys a lot of character, the maps have no scale which I feel is a failing. The authors stress that it is very important to know how many days of food the players have, but provide very little guidance on how long it should take to traverse the various terrain.
From here on out spoilers. You've been warned.
The adventure is broken into five parts.
The first bit throws the adventurers into the action, and keeps them occupied meeting several colorful characters. This is a fun different way of introducing the area, and providing one of three hooks to continue on to the rest of the adventure. Although not overtly stated, probably the most important part of this is to introduce the fact that the area is flooded, and everyone is in bad shape.
The next parts involve the travel up river, then the dam, and the drained lake. All these environments are probably very alien to the players. The flood has radically changed the "normal" of the path up river. Things from the lake now traverse the area. Everything is flooded necessitating travel by boat or a very wet walk for a very long time. The dam takes the normal dungeon crawl idea and throws it on its side because almost everything has been upturned by the water- traps are already sprung and locked doors are opened. The drained lake again presents an alien atmosphere of an ecology turned on its head.
and then into the observatory.
I would hope that the PCs would take note of the fact that someone built a dam to keep this place inaccessible under water. Of course that won't keep them out.
Here is a great opportunity to explore the remains of the observatory and the dark elves that once occupied it.
Finally the adventure ends with a timeline of what happened in the past and what will happen if the PCs don't get involved. So really you could throw this adventure hook at them and have them completely ignore it and then have it come back to bite them in the rear years later in your campaign.
The timeline tracks the progress of another adventuring party interested in the observatory as well as a witch that is an encounter earlier. With the unfolding of these two parties without PC involvement is really becomes evident that the tracking of time is important in this adventure. The ration tracking is just as important as there isn't an easy way to re-stock these items and a party may end up quitting the adventure or starving to death if not prepared.
Not every answer is given in the adventure, some just don't get answered. What caused the dam to brake, and where did the dark elves go is kind of beyond the scope of the adventure, but may be something to address in your own running of the game.
[5 of 5 Stars!]