As he was told, Steve went to the deepest part of a cave to look for diamonds. None were in view though. The voice suggested to mine. It positioned Steve to a certain position. By the looks of it, he was one block above the height that lava appears.
For the first 20 blocks travelled underground, Steve found plenty of iron. "Ooh, more iron" said the voice. Steve also found some red dust. The voice said that the red dust is a bit too complicated to use
100 blocks later, Steve found some lapis lazuli and some gold. The voice said that it was very important to keep four ingots for boots. Steve kept on mining in a straight line for a few more blocks to find more diamonds.
The voice advised Steve to be very careful when mining the diamonds. Collecting every item before mining the next ore. Steve mined two ores in front of him, walked under a third ore, looked up and mined it.
The next few events happened pretty fast, a green four legged creature fell down and exploded. Steve ressurected at his bed. There was inaudible screaming and keyboard slapping instead of hearing the voice.
The voice calmed down and headed back to the cave, into the strip mine and back to where Steve died. All his stuff was ok. Steve had mined into what looked like a huge underground cut into the earth. Like a ravine or something but underground.
Steve mined the other 5 diamonds and fled to the surface immediately because there were so many hostiles.