Dig in the dark is a gas. Baby, can you dig it?
Wizards of the Coast has posted up some Unearthed Arcana content for Dungeons & Dragons to help you modify your character to operate in the world beneath, the Underdark. I'm guessing a torch would be handy. But that's already in the PHB, I'd assume.
The new features include new fighting styles for fighters, paladins, and rangers. There's the Deep Stalker ranger archetype, the shadow sorcerer, and the warlock pact of the Undying Light also available to try out.
These are effectively "beta" rules. So feel free to use them in your games, but they aren't "official use" in any official adventures or continued play environments (aka, they're not "tourney legal"). The rules will be refined and later released in a full supplement along with other various rules.