Elf Cave is a limited time building available during the Christmas 2014 Event. This building is unlocked after completing Jingle Bell Wreck Part 2. Once built the player then began doing Santa's Tasks for prizes. If it wasn't unlocked in Part 1 of the Event, then the player was prompted to build it during Part 2 of the Event.
After December 24, 2014, the Elf Cave now earns 4 Elves as income, for the player to catch and deport (using the Elf Cannon).