Bunker Down is a mod that spawns the players inside of an underground bunker to encourage underground living. The mod adds several new blocks, new game rules, new structures, and a new plant.
To accommodate underground living, this mod adds the Underroot Plant, which grows on several types of dirt, gravel, and stone, and can occasionally drop logs when fully harvested.
If you'd rather explore the surface and ditch your starting bunker, there are other abandoned bunkers scattered throughout the Overworld. They can generate in any biome, and offer interesting design and loot.
This mod is meant to be played with other mods to encourage the player to set up bases underground. Settings of the mod can be customized in the gamerules.