I played this game for almost a week when the Tunnelers' Abyss main server was down, and it can definitely be quite addicting at times! Learning 3D minesweeper itself was a bit of a challenge, it's quite a bit harder than 2D minesweeper. But once you get the hang of it, you'll only get faster and faster. A similar 3D minesweeper game can be accessed at http://egraether.com/mine3d/ which helped me to understand it a bit better.
Right-click a block to mark it as a bomb, dig it if you think it isn't one.
Sometimes you can come to a place where there is no continuation without a 'guess' (or at least I thought so), especially on corners - you'll just have to take your chances, or find a different place where there is a solution. Large "cave"-like expanses can open up where bombs don't occur, creating some 2D-like environments where solving is particularly easy, and a large number of solve-able 'exits'.
Highly recommended if you're looking for a mind challenge. Now we just need 4D chess!