The Technic Platform is a resource that allows players to play extensively modified versions of Minecraft.  Accessible to all major platforms (PC/Mac/Linux), players can play various official mod packs including Big Dig.  It also includes Tekkit, Tekkit Lite, Tekkit Classic, Voltz and Hack/Mine.

Originally, the Technic Platform proided access to Technic (a off-line only mod combination that had many popular mods that didn't play nice in multiplayer), Tekkit (multiplayer-friendly mods), Yogbox, and other mod combinations made popular through YouTube videos. 

When updating to 1.4.7, a new feature was added allowing players to create their own Custom Mod Pack combination which could include hand-picked maps, configs, textures, etc.  From this new feature, the Big Dig Pack was was born.  Starting as one of these combinations, it became the top pack in only a few short weeks and eventually became an official part of the Technic Platform.

You need the Technic Platform in order to access Big Dig.  Get it here!