The Underdark chapter is another fairly short chapter. The first page is dedicated to describing it; a place that lies beneath the earth, but just as varied and magical as the lands that lie above it. The Underdark of the 4e world is no mere series of empty caves; it has forests, mountains, seas and cities, and these too are touched by the raw magic of creation. Bodies of fluid may be made of magma, acid, churning earth or coruscating energy. Light comes from luminescent crystals, light-lined lichen, blazing rivers of lava and jets of weirdly colored fire that explode from the ground. Its caves wend through solid ice, open into volcanic wastelands, and are soaked by dripping water that feeds underground swamps.
Our first subchapter is Underdark Denizens, which covers in brief the major races of the Underdark. These consist of the Drow (slave-takers and murderers, worshippers of the Spider Queen Lolth who consider poison an art form), Troglodytes (savage and brutish reptilians whose primitive tribes are ready sources of slave-muscle for others), Mind Flayers (parasitic brain eaters who dream of reclaiming their fallen empire), Kuo-toas (pallid, mad things that build strange temples in the deep, worshipping aboleths as living gods), Aboleths (mysterious heralds of the Far Realm) and Myconids (paranoid and secretive living mushrooms).
Next, we have the topic Under Where?, which emphasizes that the Underdark is and should be considered part of the "fantastical" world of D&D. Realism isn't needed here - in fact, it's actively a hindrance. Continent sized caverns? Rifts in the surface that open the two worlds to each other? The Underdark is heir to a large pulp fantasy tradition of subterranean worlds, and should exploit that. In particular, this is why the designers decided to flip the previous traditional idea that the Underdark should be difficult to reach and effectively gated to high levels. This, in turn, gave rise to the idea that the Underdark is divided into "depth zones", which will come into play in the later 4e Underdark sourcebook.
How Dark? likewise encourages the DM reading this book to incorporate all manner of fantastical light sources into Underdark adventures, rather than forcing players to grope along with torches and darkvision. Examples of Underdark light sources consist of glowing fungus, crystals that glimmer with an inner light, rivers of lava, naturally occurring cascades of radiant energy, multicolored flames above gas vents, or even magma glowing behind crystal veins that run in the ceiling overhead. That's not to say that the Underdark should be as bright as a summer day, but it cautions the DM to lean towards "fun" over "realism".
We close with the subtopic Underdark Locations, which mentions three examples of the fantastical locales once can visit in the deep: the ancient Vault of the Drow, the mountain-sized hollowed stalactite city of Gar Morra, and the moving fungal fortress of Hrak Azuul.
https://www.realmshelps.net/faerun/underdark/geography.shtml