It's called DUNGEONS and Dragons, yet how much thought do most of us put into what lies underground beyond traps to circumvent, monsters to kill and treasure to loot? What's going on down there when there are no adventurers poking around and generally making nuisances of themselves? Now is a chance to find out... with notes on the ecology of the subterranean world and the particular challenges that those creatures living there face, as well as more detailed information on several different types of underground space.
From there, we move on to four chapters which cover four distinct underground environments: caverns, catacombs, mines and sewers. For each there are notes on what makes that particular environment distinctive and detailed accounts of the plants and animals to be found there. There are sections on the rocks and minerals you can find there (of particular note in the mines chapter, but you never know what you might find elsewhere), the hazards to be faced and a collection of monsters.
Next comes a chapter of New Equipment. This includes useful items like flameless means of illumination (to avoid setting off explosive gases) and hip waders... and even a mask to guard against the horrible smells to be found in the likes of sewers. There are a few magic items that might come in handy as well, and a few new minerals and other materials are introduced. The final chapter is a collection of spells with an underground theme, provided for all magic-using classes. Most look pretty useful for quite mudane tasks such as detecting poisonous gases or potentially useful minerals or even shoring up a roof that looks as if it might come down. Or you may prefer to turn an enemy into a pillar of salt!
It's all written in a fairly academic style, but really empowers you to turn the 'dungeons' of your game into alternate realities in their own right rather than merely a backdrop to the party's killing and looting. Recommended if you like to make your campaign world as realistic as possible.