Landmark, and the new caves are the darling of the update. These underworld passages (are there any other types of caves?) offer a different type of environment to explore and exploit. Larger deposits of ore and special treasure chests are the rewards that the dev team is dangling to prospective spelunkers. Interestingly enough, these caves are only the first "phase" of several that will see these subterranian landscapes expanded.
Other additions in today's patch include underground resource detection, the ability to swim, a makeover to plant harvesting, and tweaks to trees and potions.
You can read up on the patch notes on the official forums and watch a preview video of Landmark's caves after the break!