That usually happens when you place your mine too deep. 6-12 levels above bedrock is ideal.
According to Zan's minimap, I'm at 7 on the z-axis. Sometimes the bedrock is right under my floor. But where ever this lava is coming from, it's higher than where I am... and lower too.
Heh. Did you like the invisible fence on the ice?
I had no clue what that was at first, then I destroyed a block. I put it back but I'm not sure where.