I was making a skeleton grinder at a spawn I found near bedrock. As I was making the room larger a lime green gas cloud appeared and starting spreading. It poisoned me and gave me 30 seconds or so of nausea. I backed out of the room and waited for the poison/sickness to stop, the gas spread a little bit ( probably 5x5 about ) and then a massive explosion happened. I would estimate about 10 blocks of TNT worth. Vaporized everything in the area except the bedrock.
Has anybody heard of this before? Searched google and youtube for 30 minutes at least and haven't found anything except a single person saying " Agreed! We need to get out killer gas! " on the Tekkit Legends 1.0.10 changelog comments section.
I even made a copy of the world and went to the location and mined out the same exact area, nothing at all out of the ordinary.
Edit: Upon further experimentation I figured it out. Created a copy of the world, used creative mode to duplicate the situation and it was natural gas. First investigation was done on a copied world while in creative mode. That did not trigger the gas expansion.
