You've probably heard about this already, but if you're trying to avoid any and all Borderlands 2 spoilers, look at another post. There are lots of other posts to look at, but this is the only one that shows you the game's brilliant Minecraft homage. After the intrepid explorers at G4 (opens in new tab) posted directions to the area, Game Front (opens in new tab) followed up with a video of what's inside: blocks to hack into pixel dust, a Creeper to kill, and a previously-teased Minecraft-themed character skin.
The hidden area is located in the Caustic Caverns, which are found "later in Borderlands 2." Follow the train tracks, turn right at the big door, and hop up the cliffside until you reach the Minecraft cavern. If you just want to see the easter egg, skip to 1:18 in the video (opens in new tab) .