The RPG resource review has mentioned almost everything I have to say about this module.

I'd definitely recommend using "The Doom from Below" in conjunction with "Murder of Crows":
MoC as a kind of prequel to the exploration in TDfB. They complement each other very well: MoC has enough interaction to smooth over the lack of social encounters in TDfB. (Which is understandable, considering the nature of the module.)

The finale is awesome and really something different. Highly recommended to kick off a new CoC campaign, especially in conjunction with MoC.

The adventure involves the characters investigating a mysterious hole in the ground in a remote corner of the New England woods. This work begins with detailed notes on both the geology of the area (including the sort of explanation a geologist might come up with to explain away the hole) and a timeline of events, running over several thousand years, explaining what is really going on. While this background ties in with the intended location, some notes are provided should you prefer to set the adventure elsewhere. Likewise, while written with the 1920s in mind, it will adapt readily to the 1890s or the present day.

It is left up to you to find a way to get the investigators to the mouth of this mysterious chasm, the only hint being - if you have run the adventure Murder of Crows - in the journal of the lunatic at the centre of the events therein. The adventure as written begins with the investigators arriving at the chasm for the first time, however you decide to get them there. Should this be whilst exploring the area or similar, their first task will probably be to go and get some climbing gear... assuming you find a way to make them want to climb down there! Some of the suggestions for getting characters involved will ensure that they come equipped, however.

The adventure itself falls into three parts. The first involves the descent into the hole, and as they get deeper, things get weirder. When they reach the bottom, the second part begins, which involves exploring the cavern there... and the third part is when they find something quite extraordinary and follow that up.

Even the descent can prove quite scary, and a lot of resources and hints are provided for the Keeper to make this initial part of the adventure quite terrifying. And it only gets more weird as the cavern at the bottom of the hole is investigated. The detail available to the Keeper is excellent, even a novice should find it easy to create the right atmosphere. It is also far more copious that necessary for the actual adventure, partly so that the Keeper can run the main adversary appropriately and partly in case he wishes to take the adventure further than written - while investigators are intended to escape with some unbelievable tales, some might wish to explore further or come back later with greater resources. The tools are to hand should they wish to do so, including notes on a range of possible outcomes for the adventure.

This is a well-constructed and atmospheric adventure in the classic style, which should prove entertaining for Keepers and investigators alike. It has a high degree of open-endedness and flexibility whilst providing outstanding support for the Keeper, whichever path he chooses.