Access to the main North Bore trunk requires cavers to negotiate a 200 foot long sump. Flooded most of the year, we were able to get past this fall due to near drought conditions affecting the area. This is the most comfortable part of the low air space. In places, there is a scant 3 inches of air (though the passage is nearly walking height). This is about as far as I could go with my C-8080 and T-32 flash in hand.