Minecraft wiki entry for Tips_and_tricks, associated text: When looking specifically for diamonds, your main target level is Y=11. This is right in the middle of the diamond generation range, and more important, it is just above the level of the "lava ocean" which appears erratically throughout the underground. That means that if you dig horizontally, most of the lava you encounter will be just below foot level, where you can see it in your tunnel, but it doesn't come flooding into your tunnel, and if you do fall into it you have at least a chance of jumping back out. (This will change drastically in the upcoming Version 1.18!)