While hiking the Switchback Trail toward Pisgah leading out from Mauch Chunk Lake I took a detour on an unknown trail and it lead me to the mouth of an abandon mine. It was filled with clear water like a pond. Without gear, I wasn't even thinking about venturing in. Some old piles of coal & slate near by too. The opening was about 3.5' high and maybe 10' wide, solid rock.
I walked and biked the Switchback many times but I've never stumbled upon this mine. It was pretty cool, with a cold draft of air bleeding out.

I borrowed a methanometer to check the high dead spaces in my house to see if it was accumulating, but didn't get any to register at all. I wouldn't consider hanging out by a mine opening. There were some crazed coal bed methane drillers that thought they could get some easy money by drilling into some of the old shafts and collecting the methane. For some reason, they couldn't get any of the landowners to agree to it.