User blog comment:Mrob27/The moat experiment/@comment-4681795-20111215035940/@comment-4583259-20111215042016

Yeah, they do. It happened right in front of my eyes once. Coincidentally, it had just turned to night-time and the music started playing the moment the mobs appeared. There were 4 Astronauts and 2 Brown_Mobs (which immediately exploded). I went over to the pit to make it deeper so the astronauts would be trapped, and cleaned up the edges. I didn't realize it at the time, but when I did that I picked up some blocks of the "Unknown" material, and put them down again while repairing the ice. A few weeks later after making my mapping program I discovered that Unknown was there, went back and picked up some blocks of it, thanks to creative mode I now have an infinite stack of it!