Earlier today, Sony Online Entertainment released a fairly major update for Landmark. This update includes a new underground cave system, new monsters both on the surface and in the caves, new equipment, and much more.
Caves and CavernsMonsters Above & Below!New EquipmentCollectible RecipesSalvagingTransportationGraveyards!

You can always find out more on Landmark, including how you can gain access to the Landmark beta over at the official site for Landmark.